5 U.S.C. § 5705: Advancements and deductions

An agency may advance, through the proper disbursing official, to an employee entitled to per diem or mileage allowances under this subchapter, a sum considered advisable with regard to the character and probable duration of the travel to be performed. A sum advanced and not used for allowable travel expenses is recoverable from the employee or his estate by—(1) setoff against accrued pay, retirement credit, or other amount due the employee; (2) deduction from an amount due from the United States; and (3) such other method as is provided by law.