Welcome to BankersOnline! Also welcome to the frustrating world of Reg E and VISA rules. As you correctly point out, it is possible that the customer's card was taken without her knowledge and returned to her, possibly by a family member. (I've seen children, parents, siblings, and roommates all do something like this). I would opine that you do not have sufficient grounds to deny the claim.
Current VISA rules prohibit chargebacks for card present transactions. An alternative for your investigation would be to file a retrieval request with the merchant to compel them to produce a copy of the signed sales receipt so you can compare the customer's signature to the sales receipt. You may also request (not require) that the customer contact law enforcement to see if they can aid in obtaining video surveillance of the transaction to help identify a suspect or the bank can attempt to contact the merchant directly to see if they will aid in the investigation.
Finally, if you conclude that the customer's card was actually used for the transaction, but not by the customer and the signature on the receipt does not match the signature on the card, you may have the right to file a Pre-Compliance case against the merchant for failure to compare signatures. VISA would require that the bank write a letter stating that it sustained as loss as the result of the merchant's failure to compare signatures, and it must include a front and back copy of the card and a copy of the sales receipt to demonstrate that the merchant did not comply with VISA Operating Rules. Your card processor should have a procedure to assist you if you choose to exercise this step.
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