Except as provided in subsections (a) and (b) regarding the acceptance of donations, the Commissioner of U.S. Customs and Border Protection and the Administrator of General Services, as applicable, may not, with respect to an agreement entered into under either of such subsections, obligate or expend amounts in excess of amounts that have been appropriated pursuant to any appropriations Act for purposes specified in either of such subsections or otherwise made available for any of such purposes.