"Can We Sue the Tribe to Enforce a Contract?"
Answer: Probably not. Tribes retain immunity from suit when conducting business transactions both on and off the reservation. Generally, a tribe can only be sued in contract if the agreement explicitly waived tribal immunity; a waiver will not be implied. Nonetheless, the U.S. Supreme Court recently held that a contractual agreement to arbitrate disputes constitutes a clear waiver of immunity.
Increasingly, tribes will agree to limited waivers of immunity. Some tribes set up subordinate entities whose assets, the tribes acknowledge, are not immune from suit, levy, or execution (although assets not held by the entity remain protected by immunity).
So, if you are asked to sue a tribe for breach of contract, you should first consider the entity with which your client contracted--i.e., a tribe, which is likely immune from suit; or a subordinate entity, for which the tribe may have waived its immunity. If you are asked to create a contract with a tribe, you must explain to your client that there may not be any remedy available in the event of a contractual breach. You should then negotiate with the tribe to reach a meeting of the minds with respect to the immunity issue. Again, some tribes will agree to a limited waiver.