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.