Final answer:
To verify a supplier's invoice, compare it to the purchase order and check the receiving report to ensure the quantity billed is accurate.
Step-by-step explanation:
To verify the quantity being billed for on a supplier's invoice, the internally generated document you should compare it to is the purchase order. This document outlines the items or services requested by a company and includes details such as quantity, price, and description. Additionally, upon receiving the goods or services, a receiving report is often completed to confirm what was received matches the purchase order. Comparing these two documents verifies that the invoice is for the correct amount and that the goods or services were delivered as expected.