Except as otherwise provided in this act and rules and regulations adopted thereunder:
(a) Every contract subject to the approval of the attorney general shall be signed by the administrative head of the affected state agency. No such contract shall be valid or effective without the approval and signature of the director of purchases.
(b) All other purchase orders and contracts issued or entered into by the division of purchases shall be signed by the director of purchases. Such purchase orders shall show on their face that an appropriation fund or allotment has been encumbered for the full amount of the liability.