31 U.S.C. § 1502: Balances available

(a) The balance of an appropriation or fund limited for obligation to a definite period is available only for payment of expenses properly incurred during the period of availability or to complete contracts properly made within that period of availability and obligated consistent with section 1501 of this title. However, the appropriation or fund is not available for expenditure for a period beyond the period otherwise authorized by law. (b) A provision of law requiring that the balance of an appropriation or fund be returned to the general fund of the Treasury at the end of a definite period does not affect the status of lawsuits or rights of action involving the right to an amount payable from the balance.