Unless otherwise provided by law all sums of money which are appropriated to the public service for any fiscal period, and which are not expended during the period, shall lapse, and shall not be issued or applied in any future fiscal period to the particular service for which the appropriation has been so made, unless a contract of engagement has been made and entered into before the expiration of the fiscal period by which a liability so to issue or apply the same has been incurred, and a certified copy of which contract or engagement has been deposited with the comptroller.