50 USC 2113 - Reemployment compensation

(a) Deduction from basic pay 
An annuitant who has retired under this subchapter and who is reemployed in the Federal Government service in any appointive position (either on a part-time or full-time basis) shall be entitled to receive the annuity payable under this subchapter, but there shall be deducted from the annuitants basic pay a sum equal to the annuity allocable to the period of actual employment.
(b) Recovery of overpayments 
In the event of an overpayment under this section, the amount of the overpayment shall be recovered by withholding the amount involved from the basic pay payable to such reemployed annuitant or from any other moneys, including the annuitants annuity, payable in accordance with this subchapter.
(c) Deposit in fund 
Sums deducted from the basic pay of a reemployed annuitant under this section shall be deposited in the Treasury of the United States to the credit of the fund.