Invoice vs Purchase Order: What's the Difference and Why It Matters

The Short Version

A purchase order comes from the buyer to the seller, before the goods or services are provided. It says: "We want to buy this, at this price."

An invoice comes from the seller to the buyer, after the goods or services are provided. It says: "We provided this. Please pay."

They're the two sides of the same transaction. In most B2B deals, both documents exist — and the invoice should match the purchase order.

Why Both Documents Exist

Without a purchase order, there's no pre-agreed record of what was ordered and at what price. If the seller delivers something different, or charges more than expected, there's nothing to reference.

Without an invoice, there's no formal payment request — the buyer has no document to process through their accounts payable system.

Together, they create a clear, verifiable paper trail:

Buyer issues PO: "We want X at price Y by date Z"

Seller delivers X

Seller issues invoice referencing the PO

Buyer matches invoice to PO — do quantities, prices, and descriptions match?

If yes, buyer approves and processes payment

If no, buyer queries the discrepancy before paying

What Goes in Each Document

Purchase Order includes:

Buyer's name and address

Seller's name and address

PO number (buyer-generated)

Items ordered: description, quantity, unit price, total

Delivery date required

Delivery address

Payment terms

Authorisation from the buyer

Invoice includes:

Seller's name and address

Buyer's name and address

Invoice number (seller-generated)

Reference to the PO number

Items supplied: description, quantity, unit price, total

Invoice date and payment due date

Payment details (bank account)

Tax information if applicable

The key overlap is the PO reference number. The invoice must reference the PO number so the buyer can match them.

Three-Way Matching

Large businesses use a process called three-way matching before approving any invoice for payment:

Purchase order — what was ordered

Delivery note / goods receipt — what was actually received

Invoice — what is being charged

If all three match, the invoice is approved for payment. If there are discrepancies — wrong quantities, wrong prices, items not received — the invoice is queried before payment.

For small businesses dealing with large corporate clients, understanding this process is essential. If your invoice doesn't match their PO, it will be held up in their AP process, not paid on time, and you'll spend time resolving a paperwork problem rather than running your business.

When a PO Is Not Required

Not every transaction needs a formal purchase order:

Consumer transactions (B2C): no PO needed

Small, informal purchases below a defined threshold

One-time or ad-hoc purchases from trusted suppliers

Service relationships where the scope is defined in a contract rather than per-order

For ongoing relationships between two businesses, a PO is typically expected for each delivery or invoice period.

Common Problems with PO and Invoice Mismatches

Different quantities. You invoiced for 100 units; the PO was for 80. Query with the client before the invoice is even sent.

Different prices. Your pricing has changed since the PO was issued. Always get a revised PO before proceeding at the new price.

No PO reference on the invoice. Some buyers will simply return invoices without a PO number and ask you to resubmit. Build it into your process.

Invoice sent to wrong person. The person who ordered from you is not the person who processes payments. Find out where to send invoices and send them directly to accounts payable.

Purchase Orders in Quotation Expert

Quotation Expert includes a Purchase Orders module for managing orders to your suppliers. On the sales side, you can record client PO numbers on your invoices so the reference is always clear. Linking invoices to PO numbers prevents payment delays and keeps your B2B client relationships running smoothly.