No contract, agreement, or other obligation involving the expenditure of money shall be entered into unless the Director of Finance shall have first certified that the money required for such contract, agreement, obligation or expenditure is in the treasury to the credit of the fund from which it is to be drawn and not appropriated for any other purpose. The sum so certified shall not thereafter be considered unappropriated until the City is discharged from the contract, agreement, or obligation; but the provisions of this Section shall not be construed as prohibiting the making of contracts for the furnishing of services of public utilities for a period extending beyond a single fiscal year when such contracts are otherwise authorized by this Charter, by ordinance, or by General Law.