Mo. Rev. Stat. § 143.986: Closing agreements.

1. The director of revenue, or any person authorized in writing by him, is authorized to enter into an agreement with any person relating to the liability of such person in respect to the tax imposed by sections 143.011 to 143.996 for any taxable period.

2. Any such agreement shall be final and conclusive and, except upon a showing of fraud or malfeasance, or misrepresentation of a material fact:

(1) The case shall not be reopened as to matters agreed upon or the agreement modified by any officer, employee, or agent of this state, and

(2) In any suit, action, or proceeding under such agreement, or any determination, assessment, collection, payment, abatement, refund, or credit made in accordance therewith, shall not be annulled, modified, set aside, or disregarded.