105 ILCS 5/19-23
Whenever refunding bonds are issued, proper reduction of taxes theretofore levied for the payment of the bonds refunded and next to be extended for collection shall be made by the county clerk upon receipt of a certificate signed by the treasurer of the school district, or by the president and clerk or other corresponding officers of the school district, showing the bonds refunded and the tax to be abated.
Money which becomes available from taxes that were levied for prior years for payment of bonds or interest coupons that were paid or refunded before those taxes were collected, after payment of all warrants that may have been issued in anticipation of these taxes, shall be placed in the sinking fund account provided in Section 19--24. It shall be used to purchase, call for payment, or to pay at maturity refunding bonds and interest thereon as herein provided.
105 ILCS 5/19-23