Can I invoice clients for work completed before receiving a PO in the US?

Understanding the Question: Invoicing Before You Have a PO

If you work with businesses in the United States, you’ll eventually run into a purchase order (PO) situation. Maybe a client tells you to “start right away,” but procurement won’t issue the PO until next week. Or you finish a project and then discover the client’s internal policy requires a PO number on invoices. At that point, a natural question comes up: can you invoice for work that was completed before receiving a PO?

The practical answer is often “yes, you can send an invoice,” but whether the invoice will be paid on your preferred timeline—and whether you’ll be protected if the client disputes the bill—depends on contract terms, the client’s internal rules, and the facts showing the client authorized the work. A PO is common proof of authorization, but it is not the only proof. In the U.S., enforceable agreements can exist without a PO, and in many cases without a signed contract, as long as essential terms are clear and the parties’ conduct shows agreement.

This article explains how POs work, how U.S. contract principles apply, what risks you take by starting work pre-PO, how to protect yourself, and how to invoice in a way that improves your chances of getting paid. It also includes process tips you can implement immediately using invoice24, a free invoicing app built to handle real-world billing needs—like tracking approvals, attaching supporting documents, adding PO fields when you get one later, and keeping a clean audit trail for your client’s accounting team.

What a Purchase Order Is (and What It Isn’t)

A purchase order is a document a buyer issues to a seller to authorize a purchase. In many organizations, especially mid-size and large companies, a PO is the formal mechanism procurement uses to control spending. A PO typically includes the buyer’s name, vendor name, billing/shipping information, a PO number, item descriptions, quantities, pricing, payment terms, and sometimes references to contracts or statements of work.

But it’s important to understand what a PO does and doesn’t mean in legal terms:

A PO is often evidence of authorization. It’s a strong internal signal that the buyer has approved the spend. For the vendor, it’s a helpful piece of paperwork to show the buyer agreed to purchase the goods or services described.

A PO is not always “the contract” by itself. Sometimes the contract is a separate master services agreement (MSA), statement of work (SOW), or set of terms, and the PO is simply the internal approval document that references that agreement. Other times, the PO includes its own terms and conditions printed on the back or linked online. In certain scenarios, the PO is the offer the vendor accepts by performing.

A PO is often more about the buyer’s accounting than your right to payment. Many disputes about POs aren’t about whether the client owes you in principle—they’re about whether Accounts Payable can process the invoice without a PO number under their rules.

This is why you might be able to invoice pre-PO and still get paid, but you might also face delays or rejections until the PO is issued or retroactively “matched.”

Can You Legally Invoice Without a PO?

In the U.S., you can generally send an invoice at any time. Invoices are not magical legal instruments; they are billing documents that request payment. The bigger question is whether the client has a legal obligation to pay—and whether your invoice will be processed smoothly.

From a contract perspective, a PO is not required for an agreement to exist. A binding agreement can form through:

Written contracts (MSA/SOW, independent contractor agreement, engagement letter, etc.).

Email exchanges where scope, price, and timing are agreed upon (“Yes, please proceed at $X for Y deliverables”).

Oral agreements in many service contexts, depending on the type of service and state law requirements.

Conduct of the parties—for example, the client requests work, you deliver it, they accept and use it.

So if the client authorized the work and you performed, you usually have a basis to invoice even if a PO arrives later or never arrives. The catch is that internal procurement rules may still cause payment delays, and the client might argue that their employees lacked authority to commit the company without a PO. That becomes a fact-heavy dispute about authority and what the client communicated to you.

Why Clients Ask for a PO (and Why Payment Might Get Stuck)

Many companies have a “no PO, no pay” policy. This is usually an internal control designed to prevent unauthorized spending, not necessarily a legal rule that erases obligations. But it can still affect you because Accounts Payable may refuse to process an invoice without a PO number, even if the business team wants to pay you.

Common reasons your invoice gets rejected or delayed include:

Three-way match requirements. Some companies require the invoice to match the PO and a receipt/approval record before payment is released.

Budget controls. The PO encumbers budget. Without it, the spend might not be assigned to a cost center.

Vendor onboarding. The client might require vendor setup, tax forms, and compliance checks before issuing a PO or paying an invoice.

Authority issues. The person who asked you to start may not be authorized to commit the company, or their approval needs to be documented by procurement.

This is why “Can I invoice?” and “Will I get paid quickly?” are different questions. Invoicing is easy; collectability is where the risk lies.

When Invoicing Pre-PO Is Common and Usually Fine

There are plenty of situations where invoicing for pre-PO work is routine:

Existing relationship with a repeat client. You’ve done multiple projects, and the PO is just late due to internal processing. Payment is likely once the PO is created and matched.

Emergency or time-sensitive work. The client requests immediate action to avoid downtime or meet a deadline, and they acknowledge the PO will follow.

Work under an existing MSA/SOW. The parties already have a contract. The PO is a purchasing mechanism, not the agreement itself.

Small-dollar work. Some clients have thresholds where POs are “nice to have” but not mandatory.

In these cases, the key is documentation. If you can show you had clear authorization and the client accepted the work, you’re in a stronger position even if the PO is issued later.

When Invoicing Pre-PO Is Risky

Pre-PO work becomes much riskier when any of the following are true:

You don’t have a contract or clear written scope. Without a contract, misunderstandings about deliverables, rates, or approvals are more likely.

The client is a large company with strict procurement. Some organizations will not budge: they may require a retroactive PO, a formal exception, or even refuse to pay invoices that predate the PO.

You’re dealing with a new client. You don’t yet know whether they pay on time, whether they honor informal approvals, or whether procurement is difficult.

The work is substantial or long-running. The bigger the bill, the bigger the scrutiny, and the more likely internal stakeholders will get involved.

You were asked to “just start” by someone junior. Authority matters. If the requester can’t approve spend, you may be asked to “eat” the pre-PO work or discount it to close the gap.

The client’s “no PO, no pay” policy is stated upfront. If they told you this policy and you proceeded anyway without a written exception, you took a known risk.

Key Contract and Legal Concepts That Affect Payment

You don’t need to be a lawyer to understand the practical legal concepts behind pre-PO invoices. These are the ideas that often determine outcomes in disputes:

Authority: Who Can Bind the Company?

Businesses act through people. If an employee requests work, the question becomes whether that employee had authority to commit the company to pay.

Actual authority exists when the company has granted the employee the power to approve purchases (for example, a department head with budget authority).

Apparent authority can exist when the company’s actions lead you to reasonably believe the employee has authority—such as giving them procurement-like titles, allowing them to negotiate vendors, or having them manage projects with outside contractors.

If you’re asked to start pre-PO, it helps to confirm authorization in writing, preferably from someone with clear budget or procurement authority.

Acceptance and Use of Deliverables

Even if paperwork is messy, if the client accepts and uses your work, that strengthens your claim that payment is owed. For example, they launch your design, deploy your code, publish your copy, or use your consulting outputs in decision-making. That’s a practical indicator the work had value and was requested.

Implied Contracts and Quantum Meruit

When there’s no formal contract, courts sometimes recognize an implied contract based on conduct. Relatedly, “quantum meruit” is a concept that can allow recovery of the reasonable value of services provided to prevent unjust enrichment. These theories can matter when a PO is missing or late, but they are not a substitute for good documentation and can be expensive to pursue. Your goal should be to avoid disputes rather than rely on these remedies.

Terms and Conditions Conflicts

Sometimes the PO includes terms that conflict with your proposal or standard terms—like payment timelines, indemnity, or acceptance criteria. If you start work before receiving the PO, you might later be presented with terms you wouldn’t have agreed to. This is one reason vendors prefer an MSA/SOW with agreed terms before work begins.

Practical Guidance: How to Protect Yourself Before Starting Work

If a client wants you to start before a PO is issued, you can reduce risk with a few practical steps. These are business habits that pay off even when everything goes smoothly.

1) Get Written Authorization to Proceed

Ask for an email confirming: scope, rate or fixed fee, start date, and a clear statement that the client authorizes you to begin and will issue a PO. Keep it simple. For example: “Please confirm you authorize us to proceed with [project] at [price], and that a PO will be issued for this work.”

In invoice24, you can store that approval email as an attachment or note tied to the client record or project. When Accounts Payable asks why the invoice predates the PO, you can provide a clean approval trail.

2) Ask for the “PO in Progress” Details

If the PO will come later, ask for:

PO request or requisition number (some systems generate this before the PO).

Name of the procurement/contact who will issue it.

Expected PO issue date.

Budget/cost center to be charged (useful for internal routing).

Having these details improves follow-through and makes it easier for your client champion to push procurement.

3) Use a Short “Authorization to Proceed” Document

When projects are substantial, consider a one-page authorization document: scope summary, pricing, start date, and a signature or email acceptance from the client. It doesn’t need to be complicated, but it should clearly state the client is authorizing you to begin prior to PO issuance.

You can generate and send this as a PDF, then attach it to your invoice record in invoice24 for future reference.

4) Set a Pre-PO Cap or Milestone

A smart compromise is: “We can start immediately, but we’ll cap pre-PO work at $X or Y hours. After that, we’ll pause until the PO is received.” This limits exposure and signals professionalism.

Invoice24 helps here by tracking time or milestones (if you bill hourly or by phases) and letting you draft invoices as you go, so you always know where you stand versus the cap.

5) Clarify What Date the PO Needs to Cover

Some clients can issue a PO with a start date that covers earlier work; others cannot due to internal policy. Ask early: “Can your PO be issued retroactively to cover work starting on [date]?” If they say no, you can propose alternatives, such as a change order to an existing PO, an amendment to an SOW, or a written exception approval from procurement.

How to Invoice for Work Completed Before Receiving the PO

Let’s assume you already did the work and the PO still hasn’t arrived. What now? The goal is to invoice in a way that reduces friction for Accounts Payable while protecting your position.

1) Put the Best Available Reference on the Invoice

If you don’t have a PO number yet, include whatever identifier the client can use to route payment:

Contract number or SOW number.

Project name and internal department contact.

Requisition number (if you have one).

Name and email of the approver.

In invoice24, you can add a custom field labeled “PO Number / Requisition” and fill it with “Pending” plus the requisition reference. This makes it obvious you’re not ignoring the PO requirement—you’re waiting on it.

2) Add Clear Service Dates and Descriptions

For pre-PO work, service dates matter. Make sure the invoice line items show the period during which work was performed and describe deliverables in a way that matches what the client expects procurement to see.

For example:

“Discovery workshop and requirements summary (Service dates: Jan 5–Jan 12)”

“UI design concepts and revisions (Service dates: Jan 13–Jan 22)”

Clear dates help a client issue a retroactive PO that correctly matches the invoice.

3) Attach Supporting Documentation

Accounts Payable teams love documentation. Attach:

The client’s written authorization to proceed.

Your proposal, estimate, or statement of work.

Approval emails for milestones or deliverables.

Timesheets or progress summaries (if relevant).

Invoice24 makes this easy by letting you attach files directly to the invoice record and resend the invoice with the attachments included.

4) Use Professional Payment Terms (and Be Realistic)

You can set standard terms like Net 15 or Net 30, but if the client’s AP system won’t process without a PO, the practical due date may slide until matching occurs. A good approach is to include your terms but also include a friendly note: “PO pending—please advise if a revised invoice with PO number is required for processing.”

This keeps your invoice “live” while opening the door for the client to tell you their preferred workflow.

5) Offer to Reissue the Invoice Once the PO Arrives

Many clients will ask you to resend the invoice with the PO number added. That’s normal. The key is to keep the invoice number consistent for your records while meeting the client’s needs.

With invoice24, you can duplicate an invoice draft, add the PO number, and send a revised version without losing your audit trail. You can also keep notes explaining what changed and when it was resent.

Should the Invoice Date Be the Completion Date or the Send Date?

Most vendors date the invoice when it is issued/sent. The service dates can (and should) reflect when the work was performed. If your invoice date is much later than the service period, that’s not necessarily a problem—clients often process invoices after completion anyway. But if you are trying to match a retroactive PO, you may want to invoice promptly and clearly state service dates, so procurement can align the PO coverage period.

Be cautious about backdating invoices. In many legitimate situations, you might issue the invoice today for services performed last month, which is normal. But changing an invoice date to something other than the issuance date can create confusion in bookkeeping and, in extreme cases, raise compliance concerns. The cleanest practice is: invoice date reflects the issuance date; line items reflect actual service dates.

What If the Client Insists on “No PO, No Pay” After the Fact?

This is where documentation and negotiation matter. If the client’s AP refuses to process your invoice, you typically have a few options:

Option A: Ask for a Retroactive PO or Exception Approval

Request that your internal sponsor escalates to procurement for a retroactive PO or a one-time exception. Provide your written authorization to proceed and proof of deliverable acceptance. Many companies can do this, even if it takes time.

Option B: Convert the Work into an SOW Under an Existing PO

If the client already has an open PO with you (or can create one quickly), they may be able to amend it with additional scope or add a change order so the invoice can be matched.

Option C: Reissue the Invoice with the New PO Number

Once the PO is issued, reissue/resend the invoice referencing the PO number and ensuring the line items match the PO description and dates as closely as possible. Invoice24 helps by allowing quick edits and clean PDF output that’s easy for AP to process.

Option D: Negotiate a Compromise

If procurement refuses a retroactive PO, you may need a business solution: a discounted invoice, a split invoice where part is covered by PO and part is written off, or rolling the value into a future PO. This isn’t ideal, but sometimes it’s preferable to prolonged nonpayment.

Option E: Escalate as a Contract/Collections Matter

If you have strong evidence of authorization and acceptance, you can escalate through a demand letter, collections process, or legal counsel. This is usually a last resort because it costs time, money, and goodwill. Before escalating, consider the size of the invoice, likelihood of recovery, and the long-term value of the relationship.

How to Reduce PO Problems with Better Upfront Language

The best way to handle pre-PO invoicing is to prevent the uncertainty. If you regularly deal with PO-driven clients, add simple language to your proposals, engagement letters, or SOWs. Examples of business-friendly concepts (written here in plain English) include:

“Authorization to proceed” clause. Work begins only after written authorization (email is fine). If a PO is required, the client confirms they will issue it within a specified timeframe.

“Client delays” clause. If administrative delays (like late POs) prevent invoicing or payment processing, timelines may shift and additional costs may apply if you have to stop and restart work.

“Billing triggers” clause. Define when you invoice: on milestones, weekly, biweekly, or upon delivery, regardless of PO issuance, while agreeing to reissue invoices with PO numbers upon receipt.

“Pre-PO cap” clause. Limit the amount of work you’ll do without a PO.

These aren’t about being adversarial—they set expectations and reduce misunderstandings. When you combine this with clean invoicing in invoice24, you look organized and professional, which often leads to faster payment.

Special Considerations: Government and Highly Regulated Buyers

Invoicing without a PO can be particularly tricky when your client is a government agency, a public institution, or a heavily regulated entity with strict procurement rules. In these environments, the PO (or equivalent authorization) may be legally required for the entity to obligate funds. That doesn’t always mean you have zero recourse, but it can significantly complicate payment.

If you work in these spaces, adopt stricter controls:

Do not start work without written authorization from the authorized contracting officer or equivalent.

Make sure your scope and pricing are formally approved.

Maintain meticulous documentation of communications and deliverables.

Invoice promptly and in the exact format required.

Invoice24 supports consistent invoice formatting, custom fields, and attachment storage so you can align your invoices with the client’s required structure and reduce the back-and-forth.

How Invoice24 Helps You Handle PO-Related Invoicing Smoothly

When clients are strict about POs, your invoicing tool matters. Invoice24 is designed to make professional billing simple, even when the client’s process is complicated. Here are practical ways invoice24 helps when you need to invoice for work completed before a PO is issued:

Custom Fields for PO Numbers and Internal References

You can add a PO Number field to every invoice so clients can easily match invoices to their purchasing system. If you don’t have the PO yet, mark it as “Pending” and include a requisition number or project reference. When the PO arrives, update the invoice quickly.

Attach Proof of Authorization and Delivery

Instead of hunting through emails later, store approvals, proposals, SOWs, acceptance emails, and timesheets directly with the invoice record. When AP asks for backup, you can respond immediately with a clean, organized package.

Clear Line Items and Service Date Ranges

Invoice24 makes it easy to create detailed line items that match what procurement expects to see: deliverable-based descriptions, service periods, rates, and quantities. This reduces “invoice mismatch” rejections.

Professional Templates and Consistent Branding

Clients take invoices more seriously when they look consistent, complete, and easy to process. A well-structured invoice reduces internal friction, especially when the invoice needs escalation for retroactive approval.

Easy Resend and Revision Workflow

If the client requests a revised invoice with a PO number, you can update and resend without recreating everything from scratch. Keep a clear record of what was sent and when, which is helpful for disputes and follow-ups.

Payment Status Tracking and Organized Client Records

When payment is delayed by PO issues, tracking matters. Invoice24 helps you monitor outstanding invoices, follow up at the right times, and keep notes on who you spoke with and what they said. That organization often makes the difference between a quick resolution and weeks of confusion.

Best Practices for Following Up When a PO Is Missing

The follow-up process is where many vendors lose momentum. The goal is to be persistent without being combative. Here’s a practical follow-up cadence you can use:

Day 0 (Invoice sent): Send invoice and ask whether a revised invoice with a PO number will be needed.

Day 3–5: If no response, follow up with the project contact: “Checking whether AP needs a PO number to process this invoice.”

Day 7–10: If still stuck, ask for the procurement/AP contact and request a status update on PO issuance.

Ongoing weekly: Keep it short, keep it factual, and attach the invoice and supporting approvals each time.

Use invoice24 to log these touchpoints in your client notes so you never lose track of what’s been said or promised.

How to Prevent “Surprise” PO Requirements

One of the most frustrating scenarios is completing work and only then learning that a PO is required. You can reduce this risk with a simple intake checklist for new clients:

Do you require a PO number on invoices?

Who issues POs, and what is the typical lead time?

Is there a vendor onboarding process or tax form requirement?

What is your invoice submission method (email, portal, AP system)?

Do you require specific wording, line item structure, or service dates?

If you build this into your sales/engagement workflow, you’ll catch PO requirements before the work starts. Even a two-minute question can save weeks of payment delay later.

Common Myths About Pre-PO Invoicing

Myth: “Without a PO, the client doesn’t owe you anything.”

Reality: A PO is not the only way an obligation can arise. Authorization and acceptance can create payment obligations even if procurement paperwork is missing. The challenge is often internal processing, not whether the work had value.

Myth: “If I just put ‘PO pending’ on the invoice, they have to pay.”

Reality: Labeling an invoice doesn’t override a client’s internal controls. It helps routing and shows good faith, but you may still need the PO or an exception approval for payment.

Myth: “I should wait to invoice until the PO arrives.”

Reality: Sometimes waiting is fine, but it can also delay the entire process. Sending the invoice promptly can prompt the internal team to push procurement, and it documents your expectation of payment.

Myth: “Backdating the invoice will fix the PO mismatch.”

Reality: The best fix is matching service dates and descriptions, then referencing the PO once issued. Keep invoice issuance dates accurate and use clear service periods.

A Simple Decision Framework You Can Use

If you’re deciding whether to invoice for pre-PO work—or whether to start work pre-PO in the first place—use this framework:

1) Do you have written authorization? If yes, proceed with cautious optimism. If no, get it.

2) Is there an existing contract or relationship? Repeat clients and existing MSAs reduce risk.

3) How strict is the client’s procurement? If they are known for “no PO, no pay,” you need a cap or exception approval.

4) How large is the exposure? The bigger the invoice, the more you should insist on formalities.

5) Can you pause work without major harm? If yes, use that leverage: start small, cap pre-PO, and pause if paperwork doesn’t arrive.

What to Do Right Now If You Already Finished the Work

If the work is done and you still don’t have a PO, take these steps in order:

Step 1: Send the invoice now with clear service dates, detailed line items, and “PO pending” plus any requisition or project reference.

Step 2: Attach the approval email, proposal/SOW, and any acceptance proof. Keep it organized.

Step 3: Ask the client contact to confirm whether a revised invoice with a PO number is required for payment processing.

Step 4: If AP rejects it, request a retroactive PO or procurement exception and provide the documentation again.

Step 5: Once the PO arrives, update and resend the invoice with the PO number using invoice24 so the client can match it quickly.

Conclusion: Yes, You Can Invoice—But Manage the Process

In the U.S., you can generally invoice clients for work completed before receiving a purchase order, because a PO is not the only way an agreement can form. However, whether you’ll be paid smoothly depends on the client’s internal procurement rules and your ability to show authorization and acceptance. The safest approach is to get written authorization to proceed, limit pre-PO exposure when possible, and invoice with clear service dates and supporting documentation.

When you use invoice24 to generate professional invoices, include PO or requisition references, attach approvals, and track follow-ups, you make it easier for your client’s Accounts Payable team to process payment—even if the PO arrives late. That combination of good documentation and clean invoicing is what turns a potentially messy “no PO yet” situation into a routine administrative step rather than a payment dispute.