41 CFR 304-5.1: -5.1 Conditions for accepting non-Federal source travel payments.

Agencies may accept payment from a non-Federal source or authorize an employee and/or the employee's spouse to accept payment on the agency's behalf only when—

(a) Agencies have issued the employee (and/or the employee's spouse, when applicable) a travel authorization before the travel begins;

(b) Agencies have determined that the travel is in the interest of the Government;

(c) The travel relates to the employee's official duties; and

(d) The non-Federal source is not disqualified due to a conflict of interest under § 304-5.3.