Mont. Code Ann. § 72-6-227

Without qualifying any other statutory right to setoff or lien and subject to any contractual provision, if a party is indebted to a financial institution, the financial institution has a right to setoff against the account. The amount of the account subject to setoff is either the proportion to which the party is, or immediately before death was, beneficially entitled under 72-6-211 or, in the absence of proof of that proportion, an equal share with all parties.