All purchases made and contracts executed by the City shall be pursuant to a requisition from the head of the office, department, or agency whose appropriations will be charged, and no contract or order shall be binding upon the City unless and until the City Manager certifies that there is to the credit of such office, department or agency a sufficient unencumbered appropriation and allotment balance to pay for the supplies, materials, equipment, or contractual services for which the contract or order is to be issued. All purchases made and contracts executed by the City shall be in accordance with the requirements of the Constitution and Laws of the State of Texas.