Payment for any improvement done or performed under the provisions of this Act, to be paid for out of any special assessment or special tax levied in installments, as herein provided, may be made in the bonds herein provided for. If such first installment is not collected when payments fall due, vouchers therefor may be issued, payable out of the first installment when collected. Such vouchers shall bear interest at a rate not more than six per centum per annum nor less than four per centum per annum, payable annually, and shall be signed by such officers as may be by ordinance prescribed.