The Secretary of the Interior is authorized to execute on behalf of the United States all necessary deeds and leases to effect the purposes of sections 459r to 459t of this title. Every such deed or lease shall contain the express condition that the grantee or lessee shall use the property exclusively for public park, recreational, and conservation purposes, and the further express condition that the United States assumes no obligation for the maintenance or operation of the property after the acceptance of such deed or during the term of such lease, and may contain such other conditions not inconsistent with such express conditions as may be agreed upon by the Secretary and the grantee or lessee: Provided