The director of finance shall act as purchasing agent for the county, unless the board designates some other 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 or equipment between departments and offices; sell any surplus supplies, materials or equipment; and make such other sales as the board authorizes. He may also, with the board’s approval, (i) establish suitable specifications or standards for all supplies, materials and equipment to be purchased for the county; (ii) inspect all deliveries to determine their compliance with such specifications and standards; and (iii) sell supplies, materials and equipment to volunteer emergency medical services agencies and firefighting companies at the same 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 by ordinance or resolution, which ordinance or resolution may, notwithstanding the provisions of § 15.2-830, provide for the use of a combination purchase order-check, which check may be made valid for such maximum amount as the board may fix, not to exceed $250. Subject to such exceptions as the board provides, before making any sale the director shall invite competitive bidding under such rules and regulations as the board establishes by ordinance or resolution. He 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 supplies, materials, equipment or contractual services.