Whenever use for war or national defense purposes of the public domain or other property owned by or under the control of the United States prevents its use for grazing, persons holding grazing permits or licenses and persons whose grazing permits or licenses have been or will be canceled because of such use shall be paid out of the funds appropriated or allocated for such project such amounts as the head of the department or agency so using the lands shall determine to be fair and reasonable for the losses suffered by such persons as a result of the use of such lands for war or national defense purposes. Such payments shall be deemed payment in full for such losses. Nothing contained in this section shall be construed to create any liability not now existing against the United States.