The Commissioner of the Fiscal Service, as delegate of the Secretary of the Treasury, may waive or modify any provision of this Circular in any particular case or class of cases for the convenience of the United States or in order to relieve any person or persons of unnecessary hardship if:
(a) Such action would not be inconsistent with law or equity;
(b) It does not impair any existing rights; and
(c) The Commissioner is satisfied that such action would not subject the United States to any substantial expense or liability.