If a requisition is a request for a purchase of commodities, the spending unit shall transmit the requisition to the state budget office for the purpose of ascertaining whether it conforms to the expenditure schedule. If it does not conform, the requisition shall be returned by the state budget office to the spending unit. If it conforms, the state budget office shall transmit the requisition to the Purchasing Division of the Department of Administration for purchase in accordance with article three, chapter five-a of this code. When a copy of the purchase order issued pursuant thereto is received from the Purchasing Division by the director in accordance with the provisions of section fourteen, article three, chapter five-a of this code, the director shall ascertain whether the unencumbered balance in the appropriation concerned, in excess of all unpaid obligations, is sufficient to defray the cost of the order and, if so, shall encumber the proper account and certify the fact to the Purchasing Division and, if not, shall notify the Purchasing Division which, upon receipt of notification, shall return the requisition to the spending unit.