The department may enter into an agreement with a person relating to the liability of the person, or of a person or estate the person represents, for a tax, license fee, or excise tax for a period ending before the date of the agreement. If the agreement is approved by the attorney general, the agreement is final and conclusive and, except upon a showing of fraud or malfeasance, or misrepresentation of a material fact, the case may not be reopened as to the matters agreed upon or the agreement modified. In a suit or proceeding relating to the tax liability of the taxpayer the agreement may not be annulled, modified, set aside, or disregarded.