Since the access device was given to the cable company, I believe that this claim would be governed by Section 1005.6(b)(3) Timely notice not given/Periodic Statment. While it is true that you have the right under Reg E to withhold $50 from the provisional credit, if your investigation determines that an error occured, you must reimburse your customer the $50 since 1005.6(b)(1)(2) would not apply.
Also, since this transaction involved a 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.
Due to the MasterCard rule, you would be unable to withold the $50.00 as this would violate your contract with MasterCard and your cardholder agreement with the customer.
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