Purchase Order vs Invoice: What Each One Does and When It Fires

A purchase order is a document you (the buyer) send to a supplier to order goods. An invoice is the document the supplier sends back asking for payment. That's the whole purchase order vs invoice distinction in one line: the PO starts the deal, the invoice closes it. Everything else is detail, but the detail is where money leaks, so let's get it right.

If you buy stock, raw materials, or resale inventory, these two documents run your entire cash cycle. Confusing them, or skipping one, is how you end up paying for things you never received, or receiving things nobody approved.

Purchase order vs invoice: the core difference

A purchase order (PO) is a buyer's commitment to buy. You create it before anything ships. It says: here's exactly what I want, how many, at what price, and where to send it. Once the supplier accepts it, a PO becomes a binding contract.

An invoice is a seller's request for payment. It arrives during or after fulfillment and says: you ordered this, we delivered it, here's what you owe and by when.

Same data, opposite direction, different moment in time.

	Purchase order	Invoice
Who creates it	Buyer	Seller
Direction	Buyer to seller	Seller to buyer
When	Before goods ship	During or after delivery
Purpose	Authorize and order	Request payment
Legally	Offer to buy (binding once accepted)	Demand for payment
References	Its own PO number	The PO number it fulfills
Affects	Committed spend	Accounts payable

What's actually on each document

Both share a lot of fields, which is why people mix them up. The overlap is deliberate: the invoice is supposed to mirror the PO so you can check one against the other.

A purchase order includes:

• PO number (your reference)
• Supplier name and address
• Line items: SKU, description, quantity, unit price
• Agreed delivery date and ship-to location
• Payment terms you negotiated (Net 30 and so on)

An invoice includes:

• Invoice number and invoice date
• The original PO number it fulfills
• Line items delivered, with quantities and prices
• Subtotal, tax, shipping, total due
• Payment due date and remittance details

The key link is the PO number printed on the invoice. No PO number, no easy way to prove what you agreed to.

A worked example

Say you run a small hardware shop and you're restocking cordless drills.

1. You send Supplier A PO-1042: 50 drills at $40 each, delivery by June 30, Net 30 terms. Committed spend: $2,000.
2. Supplier A ships 48 drills (two were out of stock) and sends Invoice INV-8871, referencing PO-1042, for 48 drills at $40 = $1,920 plus $60 shipping = $1,980 total.
3. Your goods received note confirms 48 drills actually arrived and passed inspection.

Now you match three numbers: what you ordered (PO), what you were billed (invoice), and what you received (receipt). They agree on 48 units at $40, so you approve $1,980 for payment. The two missing drills never get paid for, because they never shipped and the invoice never billed them.

That check is called a three-way match, and it's the single most valuable habit in accounts payable.

The three-way match (and the formula)

The three-way match is simple arithmetic that catches most billing errors and a lot of fraud:

Pay only if PO price and quantity, received quantity, and invoiced price and quantity all agree.

For each line item, release payment when:

invoiced_qty <= received_qty and invoiced_price = PO_price (within tolerance)

If the invoice bills 50 but you received 48, you flag it. If the invoice charges $44 when the PO said $40, you flag it. Only lines that pass all three checks get released to pay. Set a small tolerance (say 2%) so a rounding cent doesn't block a legitimate payment.

Where the PO and invoice fit in the cash cycle

Here's the order things happen in procurement:

1. Requisition: someone asks to buy.
2. Purchase order: you formally order it. Spend is now committed.
3. Goods received note: stock arrives, you count and inspect it.
4. Invoice: the supplier bills you.
5. Three-way match: PO, receipt, and invoice reconcile.
6. Payment: you pay per the agreed terms.

The PO lives at step 2, the invoice at step 4. If you only ever handle invoices and never raise POs, you've deleted your control point. You're trusting that every bill is correct, which it often isn't.

Common mistakes to avoid

• Treating a proforma invoice like a real one. A proforma is a quote-shaped estimate sent before you commit. It's not a payment request and doesn't belong in accounts payable.
• Paying invoices with no PO. Fine for a one-off coffee run, dangerous for inventory. No PO means no independent record of what you agreed to.
• Letting quantities drift. Partial shipments are normal. Your system should track that PO-1042 is 48 of 50 fulfilled, and expect a second invoice for the backorder.
• Losing the PO number link. If invoices don't carry the PO number, matching turns into manual detective work.

Doing this without a pile of spreadsheets

You can run PO-to-invoice matching in a spreadsheet when you place ten orders a month. At a few hundred it falls apart: partial shipments, backorders, price changes, and multiple invoices per PO turn into a reconciliation headache.

A proper inventory system links all three documents automatically. Inventoros handles purchase orders, goods receipts, and multi-location stock out of the box, so a received quantity updates your on-hand count and sits ready to match against the supplier's invoice. It's free, open source, and self-hosted, so your purchasing data stays on your own server. See the features list for what's included, and the documentation for how POs and receiving are modeled. If you want to push orders in from an accounting tool or automate the match, the REST and GraphQL API exposes purchase orders and stock directly.

FAQ

Is a purchase order the same as an invoice? No. A purchase order is created by the buyer before goods ship and authorizes the order. An invoice is created by the seller during or after delivery and requests payment. The invoice usually references the PO number so the two can be matched.

Which comes first, the purchase order or the invoice? The purchase order comes first. You issue the PO to order goods, the supplier fulfills it, and then the invoice follows to bill you. If an invoice arrives with no matching PO, that's a red flag worth investigating before you pay.

Can an invoice exist without a purchase order? Yes. Plenty of small or ad hoc purchases are billed without a PO. But for inventory and recurring supplier relationships, a PO gives you an approval trail and a document to match the invoice against, which protects you from overbilling.

What is a three-way match? It's the check that compares the purchase order, the goods received note, and the invoice before you pay. All three must agree on quantity and price, within a small tolerance. It's the standard control for catching billing errors and preventing payment for goods you never got.