35 ILCS 625/6
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 Act, 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 has died or become incompetent, to his legal representative, as such.
If it is determined that the Department should issue a credit or refund under this Act, 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 Act 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 person 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 Act, and the amount thereof shall be applied by the Department against any tax or penalty or interest due or to become due under this Act from such assignee.
As to any claim for credit or refund filed with the Department on or after each January 1 and July 1, no amounts erroneously paid more than 3 years prior to such January 1 and July 1, respectively, shall be credited or refunded, except that if both the Department and the taxpayer have agreed to an extension of time to issue a notice of tax liability under this Act, the claim may be filed at any time prior to the expiration of the period agreed upon. 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.
Claims for credit or refund shall be filed upon forms provided by the Department. As soon as practicable after any claim for credit or refund is filed, the Department shall examine the same and determine the amount of credit or refund to which the claimant is entitled and shall notify the claimant of such determination, which amount shall be prima facie correct.
Any credit or refund that is allowed under this Section 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.
35 ILCS 625/6