41 CFR § 301-51.102
How is my transportation reimbursement affected if I make an unauthorized cash purchase of common carrier transportation?
June 9, 2020
If you are a new employee or an invitational or infrequent traveler who is unaware of proper procedures for purchasing common carrier transportation, your agency may allow reimbursement for the full cost of the transportation. In all other instances, your reimbursement will be limited to the cost of such transportation using the authorized method of payment.
[FTR Amdt. 70, 63 FR 15968, Apr. 1, 1998; 63 FR 35538, June 30, 1998]