All purchases made by the purchasing agent shall bepursuant to a written requisition from the head of the office, department,board, or agency whose appropriation will be charged, and no contract or order shall be issued to any vendor unlessand until the director of finance certifies that there is to the credit of theoffice, department, board, or agency a sufficient unencumbered appropriationbalance to pay for the supplies, materials, equipment, or contractualservices for which the contract or order is to be issued.
Alabama Legal Code