We may waive or modify any provision of the regulations in this part. We 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 the waiver would not be inconsistent with law or equity;

(b) If the waiver does not impair any material existing rights; and

(c) If we are satisfied that the waiver would not subject the United States to any substantial expense or liability.