The Indian Self-Determination and Education Assistance Act (Act) authorizes the Government and Indian tribes to enter into contracts in which tribes promise to supply federally funded services that a Government agency normally would provide, 25 U. S. C. §450(f); and requires the Government to pay, inter alia, a tribes "contract support costs," which are "reasonable costs" that a federal agency would not have incurred, but which the tribe would incur in managing the program, §450j-1(a)(2). Here, each Tribe agreed to supply health services normally provided by the Department of Health and Human Services Indian Health Service, and the contracts included an annual funding agreement with a Government promise to pay contract support costs. In each instance, the Government refused to pay the full amount promised because Congress had not appropriated sufficient funds. In the first case, the Tribes submitted administrative payment claims under the Contract Disputes Act of 1978, which