Mont. Code Ann. § 25-4-402

In an action brought by an executor or administrator in a representative capacity, a demand against the decedent belonging at the time of the decedent's death to the defendant may be set forth by the defendant as a counterclaim as if the action had been brought by the decedent in the decedent's lifetime. If a balance is found to be due to the defendant, judgment must be rendered for the balance against the plaintiff in the representative capacity. Execution can be issued upon the judgment only in a case in which it could be issued upon a judgment in an action against the executor or administrator.