All purchases made and contracts executed by the purchasing agent shall be pursuant to a written requisition from the head 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 director of finance 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 service for which the contract or order is to be issued.
Alabama Legal Code