When such a contract shall have been entered into, it shall be lawful to issue warrants upon said fund though there be at the time of such issuance no money in the fund, but in such cases the aggregate of such warrants so issued in any year shall not exceed one-half the amount of the next annual tax levy required by such contract. Such warrants shall be stamped by the county treasurer when presented to him or her for payment, to bear interest at a certain rate thereafter until paid, such rate to be the then current rate as determined by the county auditor.