Since the customer admits to providing the debit card and authorizing the transaction, it is not a Reg E claim. However, you do have an obligation under your contract as a VISA or MasterCard issuer to assist your cardholder in this situtation. Since it is not a Reg E claim, you can require additional documentation from your customer, in this case a receipt showing that the customer did pay in cash and cardholder letter or appropriate dispute form. Also VISA requires that the cardholder first attempt to resolve the dispute with the merchant before a chargeback can be filed.
Once you have the needed documentation, either file MasterCard chargeback code 4831 Transaction Amount Differs or VISA chargeback code 86 Paid by other means.
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