105 ILCS 5/19-19
The refunding bonds may be exchanged for the bonds to be refunded on the basis of dollar for dollar for the par value of the bonds, interest coupons, and interest not represented by coupons, if any, or they may be sold at not less than their par value and accrued interest. The proceeds received from their sale shall be used to pay the bonds, interest coupons, and interest not represented by coupons, if any, without any prior appropriation therefor under any budget law.
Bonds and interest coupons which have been received in exchange or paid shall be cancelled and the obligation for interest, not represented by coupons, which has been discharged, shall be evidenced by a written acknowledgment of the exchange or payment thereof.
105 ILCS 5/19-19