The Secretary is authorized to enter into an agreement in writing with any person relating to the liability of such person (or of the person or estate for whom he acts) in respect of any internal revenue tax for any taxable period.
(1) the case shall not be reopened as to the matters agreed upon or the agreement modified by any officer, employee, or agent of the United States, and
(2) in any suit, action, or proceeding, 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.