49 C.F.R. § 29.417
A Tribe does not have to continue performance of the Tribal Transportation Program (23 U.S.C. 202(b) ) or the Tribal Transit Program (49 U.S.C. 5311(c)(1) ) that requires an expenditure of funds in excess of the amount of funds included in a funding agreement. If at any time the Tribe has reason to believe that the total amount included in a funding agreement is insufficient, the Tribe must provide reasonable notice of such insufficiency to the Self-Governance Official. If the Department does not increase the amount of funds included in the funding agreement for the Tribal Transportation Program or Tribal Transit Program, the Tribe may suspend performance of the program activity until such time as the Department transfers additional funds.
49 C.F.R. §29.417