Mo. Rev. Stat. § 521.120: Suit on bond, setoff pleaded by obligor — costs, how adjudged.

In any suit on such bond, any obligor may avail himself of any setoff or counterclaim he may have against the party to whose use the suit is brought, with the same effect as if such party were the plaintiff; and if such setoff or counterclaim shall exceed in amount the damages proved in behalf of such party, judgment shall be rendered against him in favor of the defendant setting up the setoff or counterclaim for the amount of the excess and all proper costs.