Mont. Code Ann. § 7-7-4274

When all bonds of any series or issue, with the interest thereon, have been fully paid or called in for payment and there remains in the sinking and interest fund for such series or issue any amount not required for the payment of such bonds and interest, such excess amount and all amounts subsequently collected for such fund shall be transferred to the general fund of the city or town or to the sinking and interest fund of any other issue or series of bonds outstanding that the city or town council may designate.