43 CFR 20.301 - General policy.
(a) Except as specifically authorized by law, when an employee is on official duty (no leave status), all travel and accommodations shall be at Government expense and his or her acceptance of outside reimbursement for travel expenses or services in kind from private sources, either in his or her behalf or in behalf of the Government, is not allowed.
(b) Under certain circumstances, the Department may charge a fee or accept reimbursement for providing a service or thing of value to a private source when the service or thing of value provided benefits to both the Government and the particular private source (31 U.S.C. 9701). In such instances only a portion of the costs can be accepted from the private source. The Department must pay expenses associated with its usual official business and for the benefits it receives from participating in the event. The private source can be charged or may reimburse the Department for that portion of the service provided that exceeds the Department's usual expenses and the benefits to the Government. Under this provision, payments from private sources must be deposited in the U.S. Treasury unless the bureau receiving the payment is authorized by statute to accept such payments.
(c) When a bureau is authorized by statute other than 31 U.S.C. 1353 to accept gifts, and 31 U.S.C. 1353 does not apply, the travel expenses incurred by an employee directed to participate in a convention, seminar, or similar meeting sponsored by a private source for the mutual interest of the Government and the private source may be reimbursed to the bureau and credited to its appropriation. The employee shall be paid by the bureau in accordance with the law relating to reimbursement for official travel and any accommodations and goods or services in kind furnished an employee shall be treated as a donation to the bureau and an appropriate reduction shall be made to the employee's reimbursement (46 CG 689 (1967)).
(d) When participation at a function is not in an official capacity, an employee may accept reimbursement of travel and accommodation expenses from a private source, provided that such acceptance is permitted by law and Federal regulations. Participation as a private citizen must occur on one's own time, such as while on leave. If participation should occur during the course of official travel (i.e., evening or weekend hours during official travel status), the travel voucher submitted for Government reimbursement of official duty expenses must be adjusted to claim only that per diem and travel attributable to official duty. Employees who are in positions for which the rate of pay is specified in 5 U.S.C. 5311-5318 (the Executive Schedule) are on 24-hour duty, and determinations of what constitutes official duty and what is private participation should be carefully made.