Three-Way Match Is Not Pricing Validation

Three-way match is one of the most common controls in accounts payable. It checks whether the invoice matches the purchase order and whether the goods were received. When it works, it prevents obvious problems like paying for items that were never delivered or paying the wrong quantity. That is valuable.

But three-way match is not the same thing as validating pricing. It can confirm that something was ordered, received, and invoiced, while still letting margin leakage slip through. The difference matters because many of the most expensive supplier invoice issues are not “mismatches.” They are “matches” that are priced wrong.

To understand why processes that look clean still miss invoice errors, the core explanation is here: https://www.3rd-armor.com/post/why-erp-systems-miss-invoice-errors

What three-way match is built to catch

Three-way match is built to answer a narrow question: does the invoice align to the PO and receipt?

That means it is great at catching:

• quantity discrepancies

• invoice lines that are not on the PO

• duplicate invoices

• obvious receiving errors

It is also helpful as a deterrent because suppliers know there is a control in place.

The problem is that this control assumes the PO is correct, and it assumes the pricing on the PO is still correct when the invoice arrives. Those assumptions are where drift enters.

The blind spots three-way match does not catch

Three-way match fails quietly when the invoice is “consistent” with the PO, but the PO itself is wrong or outdated. That happens more often than most teams realize, especially in high-volume operations where pricing changes constantly.

Here are the blind spots that create the most leakage.

1) Pricing drift that becomes the new normal

If the PO price was updated, accepted, or entered wrong, three-way match will approve the invoice because the numbers match. It does not ask whether that price is consistent with historical pricing or supplier agreements. It only asks whether the invoice matches what the system already believes.

2) Unit of measure and pack size changes

If the PO reflects a case and the invoice reflects a case, three-way match will approve it even if the case quantity changed or the supplier substituted packaging. This is one of the most common ways to overpay while everything still “matches.” A deeper breakdown of that pattern is here: https://www.3rd-armor.com/post/unit-of-measure-invoice-errors

3) Fees that are allowed but inconsistently applied

Freight, fuel, and handling can be attached to a PO or allowed as a standard line, which means three-way match can treat them as valid. But validity is not the same thing as fairness or consistency. Fee drift is one of the easiest ways for costs to climb while invoices keep getting approved. A full breakdown of surcharge traps is here: https://www.3rd-armor.com/post/freight-fuel-surcharge-invoice-traps

4) Substitutions that technically match but change cost structure

If substitutions are allowed, a replacement item may still pass matching because it was approved at the PO level. The invoice aligns, receiving aligns, and the control “works.” But if the substitution has a different unit, different packaging, or different fee structure, the cost can drift while the paperwork stays clean.

5) Humans correcting problems by updating the PO

This is the one nobody likes to talk about.

When a discrepancy is discovered, teams sometimes “fix” the PO so the invoice can be processed. That keeps operations moving, but it also trains the system to accept the wrong price as the new baseline. Once that happens, three-way match becomes a mechanism for repeating the mistake.

The simple add-on checks that catch what three-way match misses

The goal is not to throw out three-way match. The goal is to add lightweight checks that focus on patterns, not paperwork.

A practical approach is to add exception checks in four areas:

• Price variance against recent history

• Unit-of-measure changes on repeat-purchase items

• New or increased fees compared to prior months

• Supplier-specific patterns that repeat across many invoices

These checks do not require reviewing every line. They require identifying which invoices deserve scrutiny.

A quick baseline before changing the process

Before adding new controls, it helps to know whether the organization is exposed.

The fastest way to get that baseline is a short diagnostic:

Supplier Billing Risk Scorecard: https://www.3rd-armor.com/post/billing-risk-assessment

If the scorecard shows elevated risk, three-way match is likely catching the wrong category of problems. It may be preventing mismatches while pricing drift continues in the background.

Bottom line

Three-way match is good at confirming alignment.

But alignment is not pricing validation.

If the business wants to protect margin, it needs a way to detect pricing drift, pack size shifts, and fee behavior over time. Those issues can pass every control and still drain profit because they are not mismatches.

They are “approved” invoices that are quietly wrong.