Although most bills are sent to the head office for payment, each manufacturing plant maintains an imprest fund to pay for small, routine purchases. While reviewing the accounts payable at one of the plants, an auditor discovered that a trusted employee had embezzled $150,000 over the last 10 years. Using the imprest fund, the employee had processed and paid fictitious freight bills by using copies of authentic shipments for support. The employee had pocketed the duplicate freight payments. Identify a procedure that would have detected the fraud.