Minn. Stat. § 216B.06: RECEIVING DIFFERENT COMPENSATION.

No public utility shall directly or indirectly, by any device whatsoever, or in any manner, charge, demand, collect, or receive from any person a greater or less compensation for any service rendered or to be rendered by the utility than that prescribed in the schedules of rates of the public utility applicable thereto when filed in the manner provided in Laws 1974, chapter 429, nor shall any person knowingly receive or accept any service from a public utility for a compensation greater or less than that prescribed in the schedules, provided that all rates being charged and collected by a public utility upon January 1, 1975, may be continued until schedules are filed.