If any payment provided for in this Section exceeds the taxpayer's liabilities under this Act, the Use Tax Act, the Service Occupation Tax Act and the Service Use Tax Act, as shown on an original monthly return, the Department shall, if requested by the taxpayer, issue to the taxpayer a credit memorandum no later than 30 days after the date of payment. The credit evidenced by such credit memorandum may be assigned by the taxpayer to a similar taxpayer under this Act, the Use Tax Act, the Service Occupation Tax Act or the Service Use Tax Act, in accordance with reasonable rules and regulations to be prescribed by the Department. If no such request is made, the taxpayer may credit such excess payment against tax liability subsequently to be remitted to the Department under this Act, the Use Tax Act, the Service Occupation Tax Act or the Service Use Tax Act, in accordance with reasonable rules and regulations prescribed by the Department. If the Department subsequently determined that all or any part of the credit taken was not actually due to the taxpayer, the taxpayer's 2.1 % and 1.75 % vendor's discount shall be reduced by 2.1% or 1.75% of the difference between the credit taken and that actually due, and that taxpayer shall be liable for penalties and interest on such difference. [35 ILCS 120/3 ]
Ill. Admin. Code tit. 86, § 140.1420