Ala. Code § 11-43C-72

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 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.