1. (a) the case may not be reopened as to the matters agreed upon nor may the agreement be modified, by any officer, employee or agent of the State of New York; and
2. (b) in any suit, action or proceeding, such agreement, or any determination, assessment, collection, payment, cancellation, refund or credit made in accordance therewith, shall not be annulled, modified, set aside or disregarded.

Tax Law, § 171(18)

The Tax Commission is authorized to enter into a written agreement with any taxpayer, relative to the liability of such taxpayer in respect of any tax imposed by article 9-A, which agreement is final and conclusive, and except upon a showing of fraud, malfeasance or misrepresentation of a material fact: