Purchase Orders and Invoice Approval: Getting Through Accounts Payable

Invoices to large organisations are rarely refused. They get stuck — held in an approval queue because a reference number is missing, the amount does not match what was authorised, or nobody has confirmed the work was received. From the outside this looks identical to a client who will not pay. It is not, and the fix is completely different.

This guide covers what a purchase order does, how the approval process actually works, how to get a PO number before you invoice, why invoices fail matching, and how to chase one that has disappeared into a client's system.

What a purchase order actually is

A purchase order is the buyer's authorisation to spend money. It is issued by the customer to you, before the work, and it commits them — the mirror image of an invoice, which you issue to them afterwards and which commits them to pay.

It typically carries a unique PO number, the agreed scope and quantities, the agreed price, the delivery expectations, and the budget code the spend is allocated against. That last item is why the number matters so much: without it, the client's system does not know which budget your invoice belongs to, and an invoice that cannot be allocated cannot be approved.

For the tight definitional comparison between the two documents, see our purchase order vs invoice glossary entry. This guide is about the process around them.

The procure-to-pay flow

Understanding the sequence tells you exactly where an invoice can stall.

Notice that four of the eight failure points happen before you send anything. This is why chasing harder rarely works with corporate clients: the blockage is usually upstream of the invoice, and the person who can clear it is your day-to-day contact rather than accounts payable.

Getting a PO number before you invoice

Ask during onboarding, not when the invoice stalls. The question is routine and nobody will find it odd.

Before I raise the first invoice, a few admin questions so it goes through cleanly:

  1. Do you require a purchase order number on invoices? If so, could you confirm the PO number and its value?
  2. What is the exact registered entity name and billing address for invoicing?
  3. Which email address should invoices be sent to — is there an accounts payable inbox?
  4. Do you have a supplier onboarding form or portal I need to complete first?
  5. What are your standard payment terms and when is your payment run?

Happy to complete any vendor forms you need at the same time.

Two details that catch people out. First, a PO is usually raised for a specific value — once you have invoiced up to that amount, further invoices will fail until it is increased. Second, POs are often project-specific, so additional work needs its own PO or an amendment, agreed the same way you would agree a revised quote.

If their system needs an invoice-shaped document before it can raise the PO — which is common — send a proforma rather than a real invoice, so nothing is booked as owed before the work exists. Our guide to proforma vs commercial invoices covers how to format one and why it carries no tax.

Two-way and three-way matching

Accounts payable systems approve invoices by matching documents automatically. Knowing which model your client uses tells you exactly what has to line up.

Three-way matching is the usual model, and it explains the most frustrating failure mode: your invoice is perfect, the PO is valid, and nothing moves because no one ticked the box saying the work arrived. Your contact can usually clear that in thirty seconds — accounts payable cannot.

Why invoices fail approval

Almost all of these require a reissued invoice rather than a correction by email. Once an invoice is rejected in an AP system it generally cannot be edited in place, and a corrected copy sent informally will sit unprocessed indefinitely.

Invoicing against a purchase order

• Put the PO number in a prominent, labelled field — "PO Number: 4500123456" near the top, not buried in a description.
• Mirror the PO's line descriptions and quantities. Matching is often literal.
• Use the exact registered entity name from the PO, not the trading name.
• Do not exceed the PO value, including across multiple invoices.
• One PO per invoice unless they have told you otherwise.
• Send to the AP inbox, copying your contact so they can confirm receipt.

Building a template with a dedicated PO field saves reissuing invoices later — you can create a free invoice with custom reference fields, and UK suppliers invoicing corporate clients can use the UK invoice generator to carry company number and VAT details that vendor onboarding usually demands. Agencies billing several POs across clients may find the agency invoice generator easier to keep straight.

"No PO, no pay" and how to work with it

Many large organisations operate a strict policy: an invoice without a valid PO number will not be paid, regardless of whether the work was done and everyone agrees it was good. This is not aimed at you — it exists to stop unauthorised spending — but the consequence is entirely yours.

The protective habit is simple: do not start work until you have the PO number in writing. "The PO is being raised" is not the same as having one, and the gap between those two states has swallowed a great many invoices. If you must start on a promise, get the commitment in an email from someone with authority, and keep the exposure small.

This is also why deposits are usually impossible with these clients — their system cannot raise a payment against undelivered work. Milestone billing against a PO is the workable substitute, as covered in our guide to deposits and upfront payments.

Chasing an invoice stuck in approval

Chase the right person. Accounts payable can tell you the invoice's status; only your contact can unblock a missing goods receipt or an exhausted PO.

Subject: Invoice INV-2026-014 / PO 4500123456 — status query

Hello,

Could you confirm the status of the following invoice?

  Invoice:   INV-2026-014
  PO number: 4500123456
  Amount:    4,800.00
  Submitted: 2 August 2026 to accountspayable@[client].com
  Due:       1 September 2026

Specifically: has it been received and matched, and is it scheduled for a payment run? If it has failed matching, please let me know the reason and I will reissue promptly.

Thanks,
[your name]

If the invoice is genuinely being ignored rather than stuck, the escalation path is in our guide to overdue invoices — but exhaust the process explanation first. Applying late fees to an invoice that failed matching because of a typo you could have fixed does not help you.