29 CFR §20.87

1. (a)Agencies shall promptly refund to the appropriate party amounts paid or deducted under this subpart when—
   1. (1)A debt is waived or is otherwise not owing to the United States (unless refund is expressly prohibited by statute or regulation); or
   2. (2)The employee's paying agency is directed by an administrative or judicial order to refund amounts deducted from his or her current pay.
2. (b)Refunds do not bear interest unless required or permitted by law or contract.