25 U.S.C. § 177

No purchase, grant, lease, or other conveyance of lands, or of any title or claim thereto, from any Indian nation or tribe of Indians, shall be of any validity in law or equity, unless the same be made by treaty or convention entered into pursuant to the Constitution. Every person who, not being employed under the authority of the United States, attempts to negotiate such treaty or convention, directly or indirectly, or to treat with any such nation or tribe of Indians for the title or purchase of any lands by them held or claimed, is liable to a penalty of $1,000. The agent of any State who may be present at any treaty held with Indians under the authority of the United States, in the presence and with the approbation of the commissioner of the United States appointed to hold the same, may, however, propose to, and adjust with, the Indians the compensation to be made for their claim to lands within such State, which shall be extinguished by treaty.