If the Secretary determines that a United States direct loan is uncollectable in whole or in part or is collectable only at an unreasonable cost, or when such action would be in the best interest of the United States, the Secretary may cancel, adjust, compromise, or reduce the amount of any loan made from the revolving loan fund. The Commissioner may adjust, compromise, subordinate, or modify the terms of any mortgage, lease, assignment, contract, agreement, or other document taken as security for loans. The cancellation of all or part of a loan shall become effective when signed by the Secretary.