Mont. Code Ann. § 7-14-4723

The governing body may provide for receiving payment of the installments of the assessment before they become due and using the proceeds thereof in redeeming such bonds as may be presented for redemption by the owners thereof or for investing the proceeds in improvement bonds for other work or other satisfactory investment, but no investment of such funds shall be made so as to prejudice the prompt payment of the bonds on the date they become due.