22 CFR §1506.21

Any amount advanced to an employee for allowable travel expenses but not used for such purposes is recoverable from the employee, in accordance with 5 U.S.C. 5705, by salary offset without regard to the due process provisions in § 1506.22. This section does not apply to debts where collection by salary offset is explicitly prohibited by another statute. Collection of debt by salary offset will be in accordance with 5 U.S.C. 5514.