5 C.F.R. § 630.1705
(b) Interpretation. For the purpose of applying paragraph (a) of this section—
(4) The date paid parental leave concludes is—
(f) Application of reimbursement requirement.
(2) The determination to impose the reimbursement requirement is at the agency's sole and exclusive discretion, except that an agency may not impose the requirement if, in the agency's judgment, the employee is unable to return to work for the required 12 weeks because of—