Any funds borrowed pursuant to an authorization act, and remaining unencumbered after the completion or abandonment of a project authorized to be financed pursuant to such act, or because a project authorized to be financed by such act is not timely undertaken, or upon the expiration of the period set forth in § 7416 of this title, shall be deposited in a special fund and applied, with the approval of the issuing officers, to the cost of financing any previously authorized projects. The head of the department or agency to which such funds were appropriated for a project shall determine when such project is completed or abandoned.