The Commissioner of the Fiscal Service, as designee of the Secretary of the Treasury, may waive or modify any provision or provisions of these regulations. He may do so 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:
(a) If such action would not be inconsistent with law or equity,
(b) if it does not impair any existing rights, and
(c) if he is satisfied that such action would not subject the United States to any substantial expense or liability.