The county manager of a county adopting a short form of county managership or the director of finance of a county adopting a county manager form of government shall act as purchasing agent or shall appoint and have supervision over this official. The purchasing agent shall make all purchases for the county in the manner provided by the board of county commissioners. The purchasing agent may make transfers of supplies, materials, and equipment between departments and offices and may sell any surplus supplies, materials, or equipment and make such other sales as are authorized by the board of county commissioners. With the approval of the board, the purchasing agent may establish suitable specifications or standards for all supplies, materials, and equipment to be purchased by the county and may inspect all deliveries to determine their compliance with such specifications and standards. The purchasing agent shall have charge of such storerooms and warehouses of the county as the board of county commissioners may provide. Before making any purchase or sale, the    purchasing agent shall invite competitive bidding under such rules and regulations as the board of county commissioners may establish by ordinance or resolution. The purchasing agent shall not furnish any supplies, materials, equipment, or contractual services to any department or office except upon receipt of a properly approved requisition and unless there is an unencumbered appropriation balance sufficient to pay for the same.