41 C.F.R. § 301-10.143
You will not be reimbursed for any transportation cost for which you improperly use foreign air carrier service. If you are authorized by your agency to use U.S. flag air carrier service for your entire trip, and you improperly use a foreign air carrier for any part of or the entire trip (i.e., when not permitted under this regulation), your transportation cost on the foreign air carrier will not be payable by your agency. If your agency authorizes you to use U.S. flag air carrier service for part of your trip and foreign air carrier service for another part of your trip, and you improperly use a foreign air carrier (i.e., when neither authorized to do so nor otherwise permitted under this regulation), your agency will pay the transportation cost on the foreign air carrier for only the portion(s) of the trip for which you were authorized to use foreign air carrier service. The agency must establish internal procedures for denying reimbursement to travelers when use of a foreign air carrier was neither authorized nor otherwise permitted under this regulation.
41 C.F.R. §301-10.143