Request for authorization for reimbursement under actual expense should be made in advance of travel. However, subject to your agency's policy, after the fact approvals may be granted when supported by an explanation acceptable to your agency. Also, your agency can issue a blanket actual expense authorization under § 301-70.201.
[FTR Amdt. 70, 63 FR 15961, Apr. 1, 1998, as amended by FTR Amdt. 2011-03, 76 FR 55275, Sept. 7, 2011]