(a) An agency may authorize employees to exceed subsistence allowances established by this subtitle and by the Secretary of Defense as long as—

1. (1) The agency authorized acceptance of payment from a non-Federal source for such allowances;
2. (2) The non-Federal source pays the full amount of the subsistence expense, as authorized by the agency; and
3. (3) The subsistence expense paid by the non-Federal source is comparable in value to that offered to or purchased by other meeting attendees.