The Secretary of the Interior is authorized to cause condemnation proceedings to be instituted in the name of the United States under the provisions of section 3113 of title 40, to acquire title to the lands, interests therein, or rights pertaining thereto within the said Fredericksburg and Spotsylvania County Battle Fields Memorial, authorized to be acquired in section 425 of this title, and the United States shall be entitled to immediate possession upon the filing of the petition in condemnation in the United States District Court for the Eastern District of Virginia: Provided, That when the owner of such lands, interests therein, or rights pertaining thereto shall fix a price for the same, which in the opinion of the commission, referred to in section 425c of this title, and the Secretary of the Interior, shall be reasonable, the Secretary may purchase the same without further delay: Provided further, That the Secretary of the Interior is authorized to accept on behalf of the United States, donations of lands, interests therein or rights pertaining thereto required for the said Fredericksburg and Spotsylvania County Battle Fields Memorial: And provided further, That no public money shall be expended for title to any lands until a written opinion of the Attorney General shall be had in favor of the validity of title thereto.