The director of finance shall act as purchasing agent for the county, unless the board designates another officer or employee for such purpose. The director of finance or the person designated as purchasing agent shall make all purchases, subject to such exceptions as the board allows. He may transfer supplies, materials and equipment between departments and offices; sell, exchange or otherwise dispose of any surplus supplies, materials or equipment; and make such other sales, exchanges and dispositions as the board authorizes. He may, with the approval of the board, establish suitable specifications or standards for all goods, services, insurance and construction to be procured for the county; inspect all deliveries to determine their compliance with such specifications and standards; and sell supplies, materials and equipment to volunteer emergency medical services agencies at the same cost as the cost of such supplies, materials and equipment to the county. He shall have charge of such storerooms and warehouses of the county as the board provides. All purchases shall be made in accordance with Chapter 43 (§ 2.2-4300 et seq.) of Title 2.2 and under such rules and regulations consistent with Chapter 43 of Title 2.2 as the board establishes. He shall not furnish any goods, services, insurance or construction 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 them.