31 USC 3324 - Advances
(a) Except as provided in this section, a payment under a contract to provide a service or deliver an article for the United States Government may not be more than the value of the service already provided or the article already delivered.
(b) An advance of public money may be made only if it is authorized by
(1) a specific appropriation or other law; or
(2) the President to be made to
(A) a disbursing official if the President decides the advance is necessary to carry out
(i) the duties of the official promptly and faithfully; and
(ii) an obligation of the Government; or
(B) an individual serving in the armed forces at a distant station if the President decides the advance is necessary to disburse regularly pay and allowances.
(c) Before the Secretary of the Treasury acts on a requisition for an advance, the Comptroller General shall act on the requisition under section 3522 of this title. The Comptroller General does not countersign a requisition for an advance.
(d) The head of an agency may pay in advance from appropriations available for the purpose
(1) to the Secretary of the Army, charges for messages sent by the Secretary of the Army for the head of the agency, including charges for
(A) payment of tolls of commercial carriers;
(B) leasing facilities for sending messages; and
(C) installing and maintaining facilities for sending messages; and
(2) charges for a publication printed or recorded in any way for the auditory or visual use of the agency.