If a consumer requested $100 at an ATM but got 4-$20 bills you’d have to see the dispense error on the ATM records. If the consumer says they were charged more than was authorized, you’ll need to find out the details, what was the order for, the cost, and what was charged and why, from the vendor. Your customer may not have specific proof, or they may. Compare it to what is shown online (if available) as to the costs and what the vendor says was authorized. Remember, the burden of proof is on you.