41 C.F.R. § 301-10.123

(a) Employees may keep airline compensation for voluntarily vacating a seat under two conditions:

(1) Voluntarily vacating the seat will not interfere with performing official duties; and

(2) Any additional travel expenses resulting from vacating the seat are personally borne and not reimbursed by the Government.

(b) If volunteering causes travel delays during duty hours, the agency will charge the employee annual leave for the additional hours.