The department or any person authorized in writing by the department may enter into a written agreement with a taxpayer relating to the liability of the taxpayer, or relating to the liability of the person or estate for whom he acts, in respect of any tax administered pursuant to this article for any taxable period. If an agreement is approved by the department within the time stated in the agreement, or later agreed to, it is final and conclusive, except on a showing of fraud, malfeasance or misrepresentation of a material fact. The case shall not be reopened as to the matters agreed on or the agreement modified by any officer, employee or agent of this state. In any suit, action or proceeding, the agreement, or any determination, assessment, collection, payment, abatement, refund or credit made pursuant to the agreement, shall not be annulled, modified, set aside or disregarded.