A PO begins as purchasing intent.
Accounts Payable records what the vendor says you owe.
Start With the PO
What was ordered? Which quantities? Which cost? Which warehouse?
Then Look at the Receipt
What actually arrived, and when?
Then Compare the Vendor Invoice
If the AP invoice does not match the PO or receipt, determine whether the difference is quantity, cost, freight, timing, backorder, or another legitimate purchasing condition.
The goal is not to force three documents to look identical. It is to understand why they differ.