220 ILCS 5/9-243
No public utility, or any officer or agent thereof, or any person acting for or employed by it, shall directly or indirectly, by any device or means whatsoever, suffer or permit any corporation or person to obtain any service, commodity, or product at less than the rate or other charge then established and in force as shown by the schedules filed and in effect at the time. No person or corporation shall, directly or indirectly, by any device or means whatsoever, whether with or without the consent or connivance of a public utility or any of its officers, or employees, seek to obtain or obtain any service, commodity, or product at less than the rate or other charge then established and in force therefor. If prior to June 30, 1913, any real estate or other tangible property shall have been sold or transferred to any public utility or public service corporation, or, if before that date, any obligation of any public utility or public service corporation created in consideration of the transfer to it of any real estate or other tangible property, shall have been released or cancelled, upon consideration in whole or in part of an agreement by such public utility or public service corporation expressed in writing to render any service, or furnish any commodity or product in the future to the party or parties making such conveyance or transfer or owning such obligation, nothing in this Act contained shall be construed to in any way affect such agreement or to prevent the performance or enforcement thereof according to its terms, or to authorize the Commission to interfere with such performance or enforcement.
220 ILCS 5/9-243