18 U.S.C. § 209

(a) Whoever receives any salary, or any contribution to or supplementation of salary, as compensation for his services as an officer or employee of the executive branch of the United States Government, of any independent agency of the United States, or of the District of Columbia, from any source other than the Government of the United States, except as may be contributed out of the treasury of any State, county, or municipality; or Whoever, whether an individual, partnership, association, corporation, or other organization pays, makes any contribution to, or in any way supplements, the salary of any such officer or employee under circumstances which would make its receipt a violation of this subsection— Shall be subject to the penalties set forth in section 216 of this title . (b) Nothing herein prevents an officer or employee of the executive branch of the United States Government, or of any independent agency of the United States, or of the District of Columbia, from continuing to participate in a bona fide pension, retirement, group life, health or accident insurance, profit-sharing, stock bonus, or other employee welfare or benefit plan maintained by a former employer. (c) This section does not apply to a special Government employee or to an officer or employee of the Government serving without compensation, whether or not he is a special Government employee, or to any person paying, contributing to, or supplementing his salary as such. (d) This section does not prohibit payment or acceptance of contributions, awards, or other expenses under the terms of chapter 41 of title 5. (e) This section does not prohibit the payment of actual relocation expenses incident to participation, or the acceptance of same by a participant in an executive exchange or fellowship program in an executive agency: Provided , That such program has been established by statute or Executive order of the President, offers appointments not to exceed three hundred and sixty-five days, and permits no extensions in excess of ninety additional days or, in the case of participants in overseas assignments, in excess of three hundred and sixty-five days. (f) This section does not prohibit acceptance or receipt, by any officer or employee injured during the commission of an offense described in section 351 or 1751 of this title, of contributions or payments from an organization which is described in section 501(c)(3) of the Internal Revenue Code of 1986 and which is exempt from taxation under section 501(a) of such Code. (g) (1) This section does not prohibit an employee of a private sector organization, while assigned to an agency under chapter 37 of title 5, from continuing to receive pay and benefits from such organization in accordance with such chapter. (2) For purposes of this subsection, the term “agency” means an agency (as defined by section 3701 of title 5 ) and the Office of the Chief Technology Officer of the District of Columbia. (h) This section does not prohibit a member of the reserve components of the armed forces, or a member of the Space Force, on active duty pursuant to a call or order to active duty under a provision of law referred to in section 101(a)(13) of title 10 from receiving from any person that employed such member before the call or order to active duty any payment of any part of the salary or wages that such person would have paid the member if the member’s employment had not been interrupted by such call or order to active duty.