If you would like to have your overpayment credited to your future Illinois individual income taxes, complete Line 39. No application is required if the Treasurer's Office can determine who made the overpayment using bank, credit card and property records.Complete the contact form page and attach a file for ONE of the following proofs of payment for each payment. Form 1099-G reports the amount of overpayment of state tax or any tax rebate you have received in a tax year. All refund claims must be substantiated with the following documentation: 1. Proof of payment of the tax. 2. Com, Treasurer Maria Pappas said today.