31 USC 1517 - Prohibited obligations and expenditures

(a) An officer or employee of the United States Government or of the District of Columbia government may not make or authorize an expenditure or obligation exceeding
(1) an apportionment; or
(2) the amount permitted by regulations prescribed under section 1514 (a) of this title.
(b) If an officer or employee of an executive agency or of the District of Columbia government violates subsection (a) of this section, the head of the executive agency or the Mayor of the District of Columbia, as the case may be, shall report immediately to the President and Congress all relevant facts and a statement of actions taken. A copy of each report shall also be transmitted to the Comptroller General on the same date the report is transmitted to the President and Congress.