5 C.F.R. § 837.302 – Agency contributions.

(a) FERS annuitants. An agency that reemploys a FERS annuitant subject to retirement deductions under § 837.301(b)(1) of this part shall make contributions, as specified in 5 U.S.C. 8423, to the Fund, based on the reemployed annuitant's pay prior to offset of annuity under the provisions of § 837.303 of this part.
(b) CSRS annuitants. An agency that reemploys a CSRS annuitant is required to make an agency contribution when—
1. (1) The annuity is suspended or terminated under the provisions of subpart B of this part; and
2. (2) The appointment is subject to CSRS deductions under the provisions of subpart B of part 831 of this chapter.