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 that they will only cut trees or underbrush or disturb or remove the soil, under such regulations as the Secretary of the Interior may prescribe, and that they will assist in caring for and protecting all tablets, monuments, or such other artificial works as may from time to time be erected by proper authority: Provided further, That if such agreements to lease cover any lands the title to which shall have been acquired by the United States, the proceeds from such agreements shall be applied by the Secretary of the Interior toward the maintenance of the park.