235 ILCS 5/8-3
If it appears, after claim therefor filed with the Department, that an amount of tax or penalty or interest has been paid which was not due under this Article, whether as the result of a mistake of fact or an error of law, except as hereinafter provided, then the Department shall issue a credit memorandum or refund to the person who made the erroneous payment or, if that person died or became a person under legal disability, to his or her legal representative, as such.
If it is determined that the Department should issue a credit or refund under this Article, the Department may first apply the amount thereof against any amount of tax or penalty or interest due hereunder from the person entitled to such credit or refund. For this purpose, if proceedings are pending to determine whether or not any tax or penalty or interest is due under this Article from such person, the Department may withhold issuance of the credit or refund pending the final disposition of such proceedings and may apply such credit or refund against any amount found to be due to the Department as a result of such proceedings. The balance, if any, of the credit or refund shall be issued to the person entitled thereto.
If no tax or penalty or interest is due and no proceeding is pending to determine whether such taxpayer is indebted to the Department for tax or penalty or interest the credit memorandum or refund shall be issued to the claimant; or (in the case of a credit memorandum) the credit memorandum may be assigned and set over by the lawful holder thereof, subject to reasonable rules of the Department, to any other person who is subject to this Article, and the amount thereof shall be applied by the Department against any tax or penalty or interest due or to become due under this Article from such assignee.
As to any claim filed hereunder with the Department on and after each January 1 and July 1, no amount of tax or penalty or interest, erroneously paid (either in total or partial liquidation of a tax or penalty or interest under this Article) more than 3 years prior to such January 1 and July 1, respectively, shall be credited or refunded. Notwithstanding any other provision of this Act to the contrary, for any period included in a claim for credit or refund for which the statute of limitations for issuing a notice of tax liability under this Act will expire less than 6 months after the date a taxpayer files the claim for credit or refund, the statute of limitations is automatically extended for 6 months from the date it would have otherwise expired.
Any credit or refund that is allowed under this Act shall bear interest at the rate and in the manner specified in the Uniform Penalty and Interest Act.
In case the Department determines that the claimant is entitled to a refund, such refund shall be made only from such appropriation as may be available for that purpose. If it appears unlikely that the amount appropriated would permit everyone having a claim allowed during the period covered by such appropriation to elect to receive a cash refund, the Department, by rule or regulation, shall provide for the payment of refunds in hardship cases and shall define what types of cases qualify as hardship cases.
235 ILCS 5/8-3