The Secretary of the Interior is authorized to enter into leases with the owners of such of the lands, works, defenses, and buildings thereon within the said Fredericksburg and Spotsylvania County Battle Fields Memorial, as in his discretion it is unnecessary to forthwith acquire title to, and such leases shall be on such terms and conditions as the Secretary of the Interior may prescribe, and may contain options to purchase, subject to later acceptance if in the judgment of the Secretary of the Interior it is as economical to purchase as condemn title to the property: Provided, That the Secretary of the Interior may enter into agreements upon such nominal terms as he may prescribe, permitting the present owners or their tenants to occupy or cultivate their present holdings, upon condition that they will preserve the present breastworks, earthworks, walls, defenses, shelters, buildings, and roads, and the present outlines of the battlefields, and