Purchase order vs invoice: what each one commits you to
A purchase order is the buyer's commitment and the point at which price, quantity, and terms are legally fixed. An invoice is the supplier's demand for payment after delivery. Understanding what each document does - and when - is how businesses stop arguing about price at the moment it is already too late to change.
A purchase order is issued by the buyer before work starts or goods are delivered. An invoice is issued by the supplier after. The purchase order - once accepted by the supplier - becomes a legally binding contract: both parties are committed to the price, the quantities, and the terms written on it. The invoice follows as a formal demand for payment against that commitment. These are not interchangeable documents, and they do not carry equal weight at the same stage in a transaction. The confusion between them - or the failure to use both - is where pricing disputes begin.
The PO is where the money is committed
When a buyer issues a purchase order, they are doing two things at once. They are requesting goods or services from a supplier, and they are committing their business to pay the agreed price if the supplier accepts. The moment the supplier confirms - by signing, sending an acknowledgment, or beginning to fulfill the order - that PO is a legally binding contract. The budget is committed. The quantity is fixed. The price is settled.
This is the decision point. Everything that follows - the delivery, the invoicing, the payment - flows from what the PO says. If the PO is vague, or if it was never issued, the question of what was agreed and at what price gets deferred to the moment the invoice arrives. That is the wrong moment to be having that conversation.
A well-constructed PO includes the agreed unit price, the quantity ordered, the expected delivery date, and the payment terms. The Zigaflow purchase orders feature lets businesses create and issue POs directly from a job or order record, which means the price agreed at quote stage carries through to the supplier purchase automatically - without manual re-entry and without the risk of a figure changing between steps.
One price tracked end to end
When a PO is generated from an accepted quote, the agreed customer price and the supplier cost are both on record before anyone has spent a day on the job. If a price disagreement arises later, there is one document to go back to.
The invoice arrives after the commitment can no longer change
An invoice is issued by the supplier after delivery - or, for service-based work, after the service has been performed. It is a demand for payment against what was already agreed in the PO. The invoice does not renegotiate the PO; it asks for the amount that was committed.
That sequence matters. Once goods have been delivered or a job has been completed, neither party can change the price without the other's agreement. If the invoice arrives at a figure different from the PO - a higher unit price, an additional line item that was not ordered, a quantity that does not match what was received - the dispute that follows is costly for both sides. The supplier has already fulfilled their obligation. The buyer has already taken delivery. The argument is happening at the worst possible moment.
For many businesses, the invoice is the first moment someone in finance looks at the transaction. If no PO was issued, or was not matched against the delivery, the invoice arrives without a counterpart to check it against. Finance teams are then asked to approve payment for something they have no documented authorization to pay.
No PO means no audit trail
When a supplier invoice arrives without a corresponding purchase order, approving it requires someone to reconstruct what was agreed verbally or by email. This creates delays, errors, and disputed payments - and leaves the business exposed if the supplier later quotes a different figure from what was actually authorized.
Why price arguments happen at invoice stage
The most common invoice dispute is a price mismatch: the invoice says one thing, and the buyer expected another. This almost always traces back to something that was not locked in at the PO stage - a verbal price that was never confirmed in writing, a quote that was accepted but not converted to a PO, or a PO that was raised without checking it against the original quote.
By the time the invoice arrives, the supplier has done the work. In their view, they delivered what was agreed. The buyer may disagree. But the moment to resolve that disagreement was before the order was placed, not after delivery.
Under UK law, if no payment date is agreed between businesses, a payment becomes late 30 days after the customer receives the invoice or the goods are delivered - whichever is later. For business transactions with an agreed payment date, that date must generally be within 60 days. Suppliers can claim statutory interest on qualifying late commercial payments. An invoice dispute that drags past those thresholds is not just a business relationship problem; it carries a legal cost. And that cost only arises because the commitment - the moment the PO was accepted - was not treated with enough care.
The same dynamic applies when a business is receiving invoices from its own suppliers. If the supplier's invoice does not match the PO that was raised, that gap needs to be resolved before payment. If there is no PO to check against, the invoice has nothing to push back on.
What three-way matching is designed to catch
Three-way matching is the standard control that connects these documents. When an invoice arrives, it is checked against the original PO (did we order this, at this price?) and against the delivery note or goods received note (did we actually receive this?). If all three agree, payment is authorized. If they do not, the discrepancy is investigated before any money moves.
This process exists precisely because the invoice is not the place to be discovering what was agreed. The PO established the agreement. The delivery note confirmed what arrived. The invoice should be confirming that the supplier is billing correctly against both. Zigaflow's invoice management connects supplier invoices to the original purchase order and delivery record, so the matching happens inside the job record rather than across email chains and spreadsheets.
Businesses that skip the PO process - relying on email approvals or verbal agreements - find that the invoice becomes the only document of record. That puts finance teams in the position of approving payments they have no structured way to verify. The result is slower payment, more disputes, and suppliers who learn, correctly, that pricing is negotiable at invoice stage because it was never fixed at PO stage. The commitment happens at the PO. The invoice is just the bill.
Sources
- Late commercial payments: charging interest and debt recoveryGOV.UK · accessed 2026-09-16
- Purchase Order vs Invoice: When and Why You Use EachTipalti · accessed 2026-09-16
- Purchase Order vs Invoice: What's the Difference?Capital on Tap · accessed 2026-09-16
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