Columbia Contracts ordinance (Charter § 42)

No contract or order purporting to impose any financial obligation on the city shall be executed, nor shall the same be valid and binding upon the city, unless it be in writing, and unless the director of finance shall first certify in writing thereon that such contract or order is within the purpose of the appropriation to which it is to be charged and that there is an unencumbered balance to the credit of such appropriation sufficient to pay therefor.