Merchant With A Possible Fraud Or Card Error Issue

We are a  state government agency.  Our business is to cite truck drivers for violations. Many of these drivers use the convenience of a credit card to post bond.   I have a situation where a MasterCard customer called this morning to inform me that the charge on his bill did not belong to him. He is not a carrier, nor does he employ drivers. We can only keep the last four numbers of the credit card in our system.  I look up transactions based on that number with a date.  The citation  and cardholder's name appearing on the documentation did not belong to the customer that called, but he was correct concerning the transaction amount and date.  His bank said there had been no alerts on his card.  Our system takes information by card swipe, so I don't see how the number could have been input incorrectly. Could the customer's card that called have been compromised and he doesn't know it? Due to PCI compliance issues, there is only so much I can do.  Have you had merchants in a similar situation. Thanks. Cleve Pierce 405-521-3526