50 ILCS 505/3
The appropriate local governmental official or agency receiving goods or services must approve or disapprove a bill from a vendor or contractor for goods or services furnished the local governmental agency within 30 days after the receipt of such bill or within 30 days after the date on which the goods or services were received, whichever is later. If one or more items on a construction related bill or invoice are disapproved, but not the entire bill or invoice, then the portion that is not disapproved shall be paid. When safety or quality assurance testing of goods by the local governmental agency is necessary before the approval or disapproval of a bill and such testing cannot be completed within 30 days after receipt of the goods, approval or disapproval of the bill must be made immediately upon completion of the testing or within 60 days after receipt of the goods, whichever occurs first. Written notice shall be mailed to the vendor or contractor immediately if a bill is disapproved.
50 ILCS 505/3