Invoice vs. purchase order: what's the difference?
Updated 2026-06-20
Purchase orders and invoices are easy to confuse because they often describe the same goods or services and look similar. The difference is who issues them and why — and getting it right keeps both sides’ books clean.
Who issues what, and when
- A purchase order (PO) is issued by the buyer, before the transaction, to formally request and commit to a purchase. It says “we want to buy this, at this price.”
- An invoice is issued by the seller, after delivering (or agreeing to deliver), to request payment. It says “here’s what you owe, please pay by this date.”
So a PO comes first and flows from buyer → seller; the invoice comes back the other way, seller → buyer.
Side by side
| Purchase order | Invoice | |
|---|---|---|
| Issued by | Buyer | Seller |
| Timing | Before fulfilment | After / at fulfilment |
| Purpose | Authorise and order the purchase | Request payment |
| Creates | A commitment to buy | A payment obligation |
How they work together
In a typical B2B flow: the buyer sends a PO → the seller fulfils the order → the seller sends an invoice that references the PO number. That PO number lets the buyer’s finance team match the invoice to an approved order and pay it without chasing approvals — which is exactly why bigger clients insist on it.
Issue a PO with the purchase order generator; on the selling side, bill against it with the invoice generator, and put the PO number on the invoice (see what to include on an invoice).
Get paid against the PO faster
When your invoice quotes the right PO number it sails through approval — and from there, Duefy chases it automatically if payment still runs late, so a clean order doesn’t end in a slow payment.