35 ILCS 115/3-40
The tax imposed by this Act shall be paid to the Department by any serviceman transferring tangible personal property as an incident to a sale of service taxable under this Act. If a serviceman has paid Service Occupation Tax to his or her supplier based upon the cost price of tangible personal property before January 1, 1990, or in error on or after January 1, 1990, the serviceman, without filing any formal claims with the Department, shall be allowed to take credit against his or her Service Occupation Tax liability based upon the selling price of that property transferred in the course of providing service to the extent of the amount of the tax so paid.
If any serviceman collects an amount (however designated) that purports to reimburse the serviceman for Service Occupation Tax liability measured by receipts or selling prices that are not subject to Service Occupation Tax, or if any serviceman, in collecting an amount (however designated) that purports to reimburse the serviceman for Service Occupation Tax liability measured by receipts or selling prices that are subject to tax under this Act, collects more from the purchaser than the serviceman's Service Occupation Tax liability in the transaction, the purchaser shall have a legal right to claim a refund of that amount from the serviceman. If, however, that amount is not refunded to the purchaser by a serviceman for any reason, the supplier or serviceman is liable to pay that amount to the Department. This paragraph does not apply to an amount collected by the supplier as Service Occupation Tax, nor to an amount collected by the serviceman as reimbursement for the serviceman's Service Occupation Tax liability on receipts or cost prices that are subject to tax under this Act, as long as the collection is made in compliance with the tax collection brackets prescribed by the Department in its rules and regulations.
35 ILCS 115/3-40