The defendant in the main action may, by a suitable pleading filed in the garnishment proceedings, set up facts showing that the debt or the property with which it is sought to charge the garnishee is exempt from execution, or for any other reason is not liable for plaintiff's claim, and if issue thereon be joined by the plaintiff, it shall be tried with the issues as to the garnishee's liability. If such debt or property, or any part thereof, is found to be thus exempt or not liable, the garnishee shall be discharged as to that part which is exempt or not liable.
Iowa Code § 642.15