31 USC 1552 - Procedure for appropriation accounts available for definite periods

(a) On September 30th of the 5th fiscal year after the period of availability for obligation of a fixed appropriation account ends, the account shall be closed and any remaining balance (whether obligated or unobligated) in the account shall be canceled and thereafter shall not be available for obligation or expenditure for any purpose.
(b) Collections authorized or required to be credited to an appropriation account, but not received before closing of the account under subsection (a) or under section 1555 of this title shall be deposited in the Treasury as miscellaneous receipts.