(c)
An advance of funds under subsection (a) of this section is recoverable by the Government of the United States or the government of the District of Columbia, as the case may be, from the employee or his estate by—
(1)
setoff against accrued pay, amount of retirement credit, or other amount due to the employee from the Government of the United States or the government of the District of Columbia; and
(2)
such other method as is provided by law.
The head of the agency concerned may waive in whole or in part a right of recovery of an advance of funds under subsection (a) of this section, if it is shown that the recovery would be against equity and good conscience or against the public interest.