Mont. Code Ann. § 85-8-622

In case of omission to assess any corporation or land that should be assessed for benefits, construction, repairs, or additional assessments or to award damages, said omitted party and the owner of omitted land may in writing agree with the commissioners of said district what the assessment should be against said land or against said corporation or what said damages should be, and such agreement shall be acknowledged and witnessed, as provided above for waivers, and be filed in the court.