Closing agreements.

The Mayor may enter into a written agreement with a person relating to the liability of the person for a tax for any taxable period. If the agreement is approved by the Mayor within the time as may be stated in the agreement, or later agreed upon, the agreement shall be final and conclusive and, except upon a showing of fraud, malfeasance, or misrepresentation of a material fact:

(1) The case shall not be reopened as to the matters agreed upon;

(2) The agreement shall not be modified; and

(3) In a suit or proceeding relating to the tax liability of the taxpayer, the agreement shall not be annulled, modified, set aside, or disregarded.