I would contact the city and provide them with a copy of the documents you signed. That should take care of this, easily.
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Send the city a copy of the document under which you and the land contract seller agreed that the LC was terminated and that the seller was back in possession. If you don't have a copy, try to get it from the seller. You should retain the document in your files.
You haven't identified the other parties involved in this question so I cannot determine whether I may have a conflict in this matter. Should it turn out that I have an attorney-client relationship with any of the other parties, my response to this question will not prevent me from continuing to represent an existing client.
Land Contract is only equitable ownership and the Seller (vendor) holds the deed. Simply change the tax registration, water and sewer bill back to the Seller by showing the city the land contract paperwork and let them know you moved. It's that simple, as my esteemed colleagues have stated. Michigan law allows you to forfeit a land contract without penalty (other that for bills, taxes or damages that accrued during the time you lived there or occupied the property). Best wishes.