Illinois Statutes Chapter 35. Revenue § 405/7. Supplemental returns; refunds

(a) Supplemental returns. If the State tax credit is increased after the filing of the Illinois transfer tax return, the person or persons required to file the Illinois transfer tax return and pay the Illinois transfer tax shall file a supplemental Illinois transfer tax return. The supplemental return shall be filed and the additional tax shall be paid in the same place and manner as provided in Section 6 of this Act. The due date for the supplemental return and for the payment of the additional tax reported in the supplemental return shall be no later than 3 months after the earliest of:

provided that if the federal transfer tax may be deferred or paid in installments, then part or all of the additional Illinois transfer tax may be deferred or paid in installments under rules consistent with subsection (b) of Section 6 of this Act.

(b) Refunds. If the state tax credit is reduced after the filing of the Illinois transfer tax return, the person who paid the Illinois transfer tax (or the person upon whom the burden of payment fell) shall file an amended Illinois transfer tax return and shall be entitled to a refund of tax or interest paid on the Illinois transfer tax. No interest shall be paid on any amount refunded.