If you determine that the cash payment was made under a non-emergency circumstance, reimbursement to the traveler must not exceed the cost which would have been properly chargeable to the Government had the traveler used a government provided payment resource, (e.g., individual Government contractor-issued travel charge card, centrally billed account, or GTR). However, an agency can determine to make full payment when circumstances warrant (e.g., invitational travel, infrequent travelers and interviewees).
41 C.F.R. §301-72.203