Answer

Client wants to add a PO number after the invoice is sent

You do not have to reissue. The clean fix is an amendment that adds the PO without changing the invoice number or restarting the payment clock. Here is how to handle it, and how to avoid the situation next time.

Let clients add PO numbers themselves Back to all answers →

When a client wants to add a PO number after the invoice has been sent, the wrong answer is to void the invoice and reissue. Reissuing creates a new invoice number, restarts the payment clock in some AP systems, and confuses your records. The right answer is to treat the PO addition as an amendment to the existing invoice. The invoice number, dates, and totals stay the same; only the PO field changes. Some invoicing tools handle this natively, with a tracked audit trail. Where the tool does not support it, you can replicate the behavior manually.

Why this happens

A PO comes in late for a few common reasons. Knowing which one applies changes how you respond:

•The client raised the PO after agreeing the work. Their procurement process is sequential: contract first, then PO, then work, then invoice. If you invoiced before the PO landed, the AP team needs the PO added retroactively to match.
•You did not know the client required a PO. Common with new clients or new engagement types. The PO requirement only surfaces when AP tries to process the invoice and rejects it.
•The wrong PO is on the invoice. A correction, not an addition. Same mechanical fix.
•The PO covers multiple invoices and the client wants to consolidate. A single PO referenced by several invoices is normal; each invoice references the same PO number.

In all four cases, the right output is the same: the invoice has the correct PO, the invoice number does not change, and there is a clean record of when and why the PO was added.

Best

Let the recipient add the PO from the invoice page directly

If your invoicing tool hosts the invoice as a page and supports recipient editing, the client adds the PO number themselves from the link you sent. The invoice updates in place. You get a notification. AP sees the corrected invoice immediately, with the same number and amounts. No reissue, no replacement file, no version confusion.

JupiterInvoice supports this natively. See recipient editing.

Workable

Amend the invoice yourself and resend the same link

If your tool lets you edit the PO field on an issued invoice (without forcing a new invoice number), update it on your side and notify the client that the invoice page now shows the PO. The link they already have continues to work; the PDF download from the same page now includes the PO. The invoice number, dates, and total are unchanged.

Keep a note of when the PO was added and by whom. AP often asks.

Acceptable

Reissue with the same invoice number, marked as a revision

If your tool will not let you amend an issued invoice without forcing a new number, the next best option is a manual reissue that keeps the original number, adds a small revision marker ("Rev 2" or "Amended: PO added"), and notes the change in the invoice itself. Send the revised invoice to the client and to AP, with a short note explaining that the change is purely a PO addition.

Most AP systems handle this fine. The risk is that the system treats the revision as a new invoice and starts the payment clock over. If you suspect that is happening, ask AP directly whether the original date or the revised date applies.

Last resort

Void and reissue with a new invoice number

The path most invoicing tools default to. Avoid it where possible. The new invoice number breaks the link with the original, often restarts the payment clock from zero, and complicates reconciliation on both sides. Use this only when the tool offers no other option and the amount or scope is also changing materially.

If you have to do this, send the void notice and the new invoice in the same message to AP, with a one-line explanation. Their system needs to know to cancel the original.

How to avoid the situation next time

→Ask before invoicing. A single sentence in your engagement email: "Do you use POs? If yes, please send the PO number before I invoice." Saves the entire round-trip.
→Default to a tool that allows recipient PO addition. The whole class of "client wants to add a PO after sending" goes away when the client can just add it themselves from the invoice link.
→Save PO numbers per client. If the same client uses the same PO across multiple invoices, store it on the customer record so future invoices include it automatically.
→For background on POs themselves, see the purchase order guide.

Adding a PO after the invoice is sent

Does adding a PO restart the payment clock?

Sometimes, depending on the AP system. Adding the PO via an amendment that keeps the original invoice number usually does not. A void-and-reissue with a new invoice number often does. The cleanest answer is to amend in place, with the original invoice date intact.

Should I change the invoice number when I add a PO?

No, if you can avoid it. The invoice number is the reference both sides use to track the document. Changing it creates ambiguity (which one is the real one?) and complicates reconciliation. Keep the same number; add a revision marker only if your records require it.

What if the client wants me to backdate the PO?

Add the PO with today's date as the amendment date and the original invoice date unchanged. Backdating the PO itself is a procurement question for the client, not you. Most companies will accept the PO added after the invoice as long as the underlying work and pricing match the PO terms.

Does AP need a new invoice file with the PO added?

Usually they want the updated invoice to show the PO. If your tool generates a fresh PDF after the amendment, send that to AP with a one-line note. If you are using a hosted-link delivery, the same link now shows the PO without you sending anything new.

What if there is no PO and the client now wants one?

Same process. Treat the PO addition as an amendment to the existing invoice. The PO needs to come from the client's procurement system (they create it, not you), but applying it to the invoice on your side is a field change, not a new document.

Let clients add the PO number themselves

JupiterInvoice gives the recipient a button on the invoice page to add or update the PO number. The invoice updates in place, you get notified, AP sees the corrected version, and the invoice number never changes. Free, no signup.

Create your first invoice

No signup required. Build now, save later.

How to forward an invoice to accounts payable

How to forward a vendor's invoice to your AP team without losing the original, and what senders can do to make AP forwar...

Client wants me to use their vendor portal: what to do

Big clients often require submitting invoices through a vendor portal. Here is how to evaluate the request, when to push...

What is a purchase order?

A purchase order is a buyer's written authorization to buy. Here's how POs work, what goes on one, and how they pair wit...

Let your customers edit their own invoice

Recipients add PO numbers, fix billing entities, and update AP contacts directly on the invoice. Every change is version...