25 U.S.C. § 5139

A tribe or tribal corporation to which a loan is made or insured pursuant to sections 5136 to 5143 of this title (1) may waive in writing any immunity from suit or liability which it may possess, (2) may mortgage or otherwise hypothecate trust or restricted property if (a) authorized by its constitution or charter or by a tribal referendum, and (b) approved by the Secretary of the Interior, and (3) shall comply with rules and regulations prescribed by the Secretary of Agriculture in connection with such loans.