The Secretary of the Interior in his discretion is authorized to lease at rates to be determined by him any State, county, or privately owned lands chiefly valuable for grazing purposes and lying within the exterior boundaries of a grazing district when, in his judgment, the leasing of such lands will promote the orderly use of the district and aid in conserving the forage resources of the public lands therein: Provided, That no such leases shall run for a period of more than ten years and in no event shall the grazing fees paid the United States for the grazing privileges on any of the lands leased under the provisions of this section be less than the rental paid by the United States for any of such lands: Provided further, That nothing in this section shall be construed as authorizing the appropriation of any moneys except that moneys heretofore or hereafter appropriated for construction, purchase, and maintenance of range improvements within grazing districts, pursuant to the provisions of sections 315i and 315j of this title, may be made additionally available by Congress for the leasing of land under this section and sections 315m-2 to 315m-4 of this title.