Purchase Orders: Why the Missing PO Number Delays Half Your Invoices

Three-way matching

A buyer with any accounts payable automation matches three documents before paying: the purchase order, which records what was ordered and at what price; the goods receipt note, which records what arrived; and your invoice, which records what you are charging.

All three must agree within tolerance. If they do, payment proceeds automatically. If they do not, the invoice is held for manual review — and manual review happens when someone has time.

Why invoices fail the match

The last one is entirely outside your control and is a common cause of long delays, which is why asking for the GRN reference is worth doing.

• No PO number on the invoice, so the system cannot find the order at all.
• Quantity variance: you delivered ninety-eight units against an order for one hundred, and invoiced for ninety-eight. Correct, but a variance.
• Price variance: a rate increase agreed by email but never reflected in the PO.
• Entity mismatch: the invoice is addressed to a group company rather than the ordering entity.
• Goods receipt not posted: the goods arrived but nobody at the buyer's warehouse recorded it.

What suppliers should do

Capture the purchase order number before starting work, not at invoicing. Put it on the invoice in a field the buyer's system can read. Invoice the exact entity named on the PO. Where quantity or price differs from the order, flag it before invoicing and get the PO amended.

For services, agree what evidence of delivery looks like — a signed timesheet, an acceptance email — and attach it.

Asking the right question when chasing

Rather than asking when payment will be made, ask whether the invoice has been matched and approved, and if not, what the exception is. That question reaches the person who can actually unblock it.

It also surfaces the real answer quickly: an invoice held for a quantity variance has a different fix from one held because the buyer is managing cash.

Frequently asked questions

### What is three-way matching?

A control where the purchase order, the goods receipt note and the supplier invoice must agree before payment is released. Mismatches beyond tolerance are routed to an exception queue for manual review rather than paid automatically.

### Can I invoice without a PO number?

You can, but with buyers who require one the invoice will usually fail the match and sit unpaid. Where the buyer operates a PO system, the number should be obtained before work begins and quoted on every invoice and delivery document.

### What if I delivered less than the PO quantity?

Invoice for what you delivered and flag the variance to the buyer in advance so the PO can be closed short or amended. An unexplained quantity variance is one of the commonest reasons an otherwise correct invoice is held.