Mont. Code Ann. § 61-4-403

It is unlawful for any manufacturer or wholesale distributor of motor vehicles to sell or enter into a contract for the sale of motor vehicles to any motor vehicle dealer on the condition or under an agreement, expressed or implied, that the dealer will finance the purchase or sale of any motor vehicle or vehicles only through a designated finance company or finance agency. Any such condition, agreement, or understanding is against the public policy of the state, and such condition, agreement, or understanding is unlawful, void, and unenforceable, either at law or equity.