25 U.S.C. § 4192: Security and repayment

enter into a contract, in a form acceptable to the Secretary, for repayment of notes or other obligations guaranteed under this subchapter; pledge any grant for which the issuer may become eligible under this chapter; demonstrate that the extent of such issuance and guarantee under this subchapter is within the financial capacity of the tribe and is not likely to impair the ability to use grant amounts under subchapter I, taking into consideration the requirements under section 4133(b) of this title ; and furnish, at the discretion of the Secretary, such other security as may be deemed appropriate by the Secretary in making such guarantees, including increments in local tax receipts generated by the activities assisted under this chapter or disposition proceeds from the sale of land or rehabilitated property. the Secretary may apply grants pledged pursuant to subsection (a)(2) to any repayments due the United States as a result of such guarantees; and grants allocated under this chapter for an Indian tribe or housing entity (including program income derived therefrom) may be used to pay principal and interest due (including such servicing, underwriting, and other costs as may be specified in regulations issued by the Secretary) on notes or other obligations guaranteed pursuant to this subchapter. The full faith and credit of the United States is pledged to the payment of all guarantees made under this subchapter. Any such guarantee made by the Secretary shall be conclusive evidence of the eligibility of the obligations for such guarantee with respect to principal and interest, and the validity of any such guarantee so made shall be incontestable in the hands of a holder of the guaranteed obligations.