Mont. Code Ann. § 7-12-4221

The council or commission of any city or town which has heretofore created or may hereafter create any special improvement district or districts for any purpose may in its discretion create, establish, and maintain by ordinance a fund to be known and designated as the special improvement district revolving fund in order to secure prompt payment of any special improvement district bonds or sidewalk, curb, and alley approach warrants issued in payment of improvements made therein and the interest thereon as it becomes due. Nothing herein shall authorize or permit the elimination of a revolving fund until all bonds and warrants secured thereby and interest thereon have been fully paid and discharged.