All purchases made and contracts executed by the City Purchasing Agent shall be pursuant to a written requisition from the chief of the office, department or agency whose appropriation will be charged, and no contract or order shall be issued to any vendor unless and until the City Treasurer certifies that there is to the credit of such office, department or agency a sufficient unencumbered appropriation balance to pay for the supplies, materials, equipment or contractual services for which the contract or order is to be issued.