Provided further, that not more than five hundred (500) acres may be purchased or donated in any one (1) county outside the existing boundaries of any one (1) existing national forest.
The provisions of this section and Section 3-5-15 shall not apply to, or be construed to prohibit, any landowner owning lands in the State of Mississippi who may desire to exchange any part or all of such lands with the United States Government, or any agency thereof, within the external boundaries of an existing national forest for an equal amount of land or lands of equal value owned by the United States Government, or any agency thereof.