We received an ATM fraud claim of unauthorized withdrawal from a savings account. Upon resolution, we credited the customer the amount of fraudulent withdrawals + fees associated with them. Do we owe interest on the amount that should have been in the savings account between the time of fraud and resolution? TIA

Yes.

Comment 11(c)-6. Correction of an error. If the financial institution determines an error occurred, within either the 10-day or 45-day period, it must correct the error (subject to the liability provisions of §§ 1005.6(a) and (b)) including, where applicable, the crediting of interest and the refunding of any fees imposed by the institution. In a combined credit/EFT transaction, for example, the institution must refund any finance charges incurred as a result of the error. The institution need not refund fees that would have been imposed whether or not the error occurred.