The Secretary of the Interior is authorized to accept from grantors title to non-Federal land and interests in land, together with improvements thereon, situated within or adjacent to the Great Smoky Mountains National Park, and in exchange therefor, to convey by deed on behalf of the United States to the aforesaid grantors, land or interests therein, together with improvements thereon, situated within the Great Smoky Mountains National Park: Provided, That such exchanges may be made without additional compensation by either party to the exchange when the properties to be exchanged are of approximately equal value; however, when the properties are not of approximately equal value, as may be determined by the Secretary, an additional payment of funds shall be required by the Secretary or by t