Not only does the $50 consumer liability from 1005.6 (CFPB Reg E citation) not apply, when you file a chargeback for MasterCard, Section 1.4 of MasterCard Chargeback Guide states:
It is very important to note that when an issuer has billed a transaction to its cardholder’s account for payment and then chooses to exercise a chargeback right, the issuer must credit the cardholder’s account for the amount of the chargeback.
As Randy said, file the chargeback and provide provisional credit for the full amount of the transaction.
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