Whenever the Secretary of the Army determines that any Government-owned lands other than lands withdrawn or reserved from the public domain within reservoir areas under his control (1) are not required for project purposes or for public recreational use, and (2) are being used for or are available for cottage site development and use, he is authorized to offer such lands, or any part thereof, for sale for such purposes in accordance with the provisions of this subchapter: Provided, however, That any lands held under lease for cottage site purposes on August 6, 1956 shall not be offered for sale to anyone other than the lessee until after sixty days from the date of the written notice to the lessee as provided in section 460f of this title, or the termination or expiration date of such lease, whichever is later, and the lessee shall have the right during such period to purchase any lands leased to him which the Secretary determines are available for sale.