41 U.S.C. § 6301: Authorization requirement

A contract or purchase on behalf of the Federal Government shall not be made unless the contract or purchase is authorized by law or is under an appropriation adequate to its fulfillment. the Secretary of Defense; or the Secretary of Homeland Security with respect to the Coast Guard when the Coast Guard is not operating as a service in the Navy. Subsection (a) does not apply to a contract or purchase made by a defined Secretary for clothing, subsistence, forage, fuel, quarters, transportation, or medical and hospital supplies. A contract or purchase made by a defined Secretary under this subsection may not exceed the necessities of the current year. The defined Secretary shall immediately advise Congress when authority is exercised under this subsection. The defined Secretary shall report quarterly on the estimated obligations incurred pursuant to the authority granted in this subsection. Land may not be purchased by the Federal Government unless the purchase is authorized by law.