5 C.F.R. § 837.303 – Annuity offset.

(a) Applicability. When the right to receive annuity continues during reemployment (even though actual receipt of annuity may have been waived under 5 U.S.C. 8345(d) or 8465(a)), the pay of the reemployed annuitant shall be offset by the amount of annuity allocable to the period of reemployment, except that—
1. (1) No amount shall be offset from pay in accordance with this section for a period for which the annuitant has elected to receive FEC benefits in lieu of annuity; and
2. (2) No amount shall be offset from a lump-sum payment of annual leave, made on or after termination of the reemployment period.
(b) Payment. The employing agency shall pay to the Fund the full amount required to be offset from a reemployed annuitant's salary under this section in accordance with instructions issued by OPM. Payment in full to the Fund is not contingent on actual offset from the reemployed annuitant's salary.
(c) Computation. To compute the amount of the annuity offset for any particular pay period, divide the amount of annuity for the calendar days included in the pay period by the number of hours that would constitute a full-time tour of duty for that pay period, then multiply the result by the number of hours actually paid for the pay period, not to exceed the number of hours that constitutes a full-time tour of duty.