65 ILCS 5/7-1-31
After a part of a municipality is disconnected and annexed to another municipality, the indebtedness and liabilities of the municipality from which the part is disconnected shall be assumed and paid by the enlarged annexing municipality, in the same proportion as the taxable property in the disconnected part bears to the taxable property in the municipality from which the part was disconnected as it existed immediately before the disconnection, according to the last assessment for taxation. This duty to assume and pay a proportionate share of indebtedness and liabilities shall also apply to the indebtedness and liabilities of the school district or township in which the disconnected part was situated.
The amount of the indebtedness and liabilities to be assumed and paid by the enlarged annexing municipality shall be determined by the corporate authorities of the annexing municipality and of the municipality from which a part was disconnected, or, as the case may be, by the school authorities of the school district or township in which the disconnected part was situated.
65 ILCS 5/7-1-31