20 ILCS 2505/2505-475
When the Department, through its own error, has entered State tax on its records under the wrong designation (such as recording a use tax payment as retailers' occupation tax, or a retailers' occupation tax payment as use tax, and so forth), the Department has the power to correct the error on its records and to notify the State Treasurer of the change so that the Treasurer can make the necessary corresponding changes in the Treasurer's records in case the erroneous entry has been made in those records. If the erroneous entry in the Department's records is due to a mistake in reporting by the taxpayer and the taxpayer agrees that he or she has made a reporting error that should be corrected, the Department may correct its records accordingly and notify the State Treasurer of the change so that the Treasurer can make the necessary corresponding changes in the Treasurer's records in case the erroneous entry has been made in those records.
The Department may similarly correct (i) errors in the distribution, as between municipalities and counties, of taxes that are imposed by those municipalities and counties but collected for them by the Department as agent and (ii) errors by which State taxes are erroneously credited as municipal or county tax or by which municipal or county taxes are erroneously credited or recorded as State tax, giving notices to the State Treasurer as may be necessary to enable the Treasurer to make corresponding corrections in the Treasurer's records.
20 ILCS 2505/2505-475