5 CFR § 715.202 - Resignation.
(b) Withdrawal of resignation. An agency may permit an employee to withdraw his resignation at any time before it has become effective. An agency may decline a request to withdraw a resignation before its effective date only when the agency has a valid reason and explains that reason to the employee. A valid reason includes, but is not limited to, administrative disruption or the hiring or commitment to hire a replacement. Avoidance of adverse action proceedings is not a valid reason.