By looking at their transaction history, we have a customer who pays monthly to a credit card company. It just so happens that for some reason, either they fat fingered the payment amount or they thought they would be able to pay a bigger amount and it came through as an ACH. Now the customer is stating that it's unauthorized and wants to be reimbursed immediately. Is there anything in the ACH regs that requires us to reimburse the customer? Shouldn't the customer take it up with the credit card company? We receive a lot of these types of "unauthorized txn" requests, about 10/day and it just seems that most people have a relationship with the company they are saying its unauthorized to get their money back.

The ACH Rules allow for a right of return, and Reg E will dictate the reimbursement for the unauthorized claim.

The customer will need to sign a Written Statement Under Penalty of Perjury (the standard ACH form) to indicate that the amount debited from their account was greater than the authorized amount. The ACH system does not allow partial returns, so the return would be for the full amount.

Once the return was processed, you would then recredit the customer account and follow your normal Reg E procedures for resolution and logging the error.