Before making any purchase or sale, the Commissioner of Purchases and Supplies shall give opportunity for competition, under such rules and regulations as the Council shall establish. Supplies required by any department may be furnished upon requisition from the stores under the control of the Commissioner of Purchases and Supplies, and whenever so furnished shall be paid for by the department furnished therewith by warrant made payable to the credit of the stores account of the Division of Purchases and Supplies. The Commissioner of Purchases and Supplies shall not furnish any supplies to any department unless there be to the credit of such department an available appropriation balance in excess of all unpaid obligations sufficient to pay for such supplies.