31 U.S.C. § 1342: Limitation on voluntary services

An officer or employee of the United States Government or of the District of Columbia government may not accept voluntary services for either government or employ personal services exceeding that authorized by law except for emergencies involving the safety of human life or the protection of property. This section does not apply to a corporation getting amounts to make loans (except paid in capital amounts) without legal liability of the United States Government. As used in this section, the term “emergencies involving the safety of human life or the protection of property” does not include ongoing, regular functions of government the suspension of which would not imminently threaten the safety of human life or the protection of property.