Mont. Code Ann. § 72-34-437

Unless the trustee accounts for receipts from rental property pursuant to 72-34-435, the trustee shall allocate to income an amount received as rent of real or personal property, including an amount received for cancellation or renewal of a lease. An amount received as a refundable deposit, including a security deposit or a deposit that is to be applied as rent for future periods, must be added to principal and held subject to the terms of the lease and is not available for distribution to a beneficiary until the trustee's contractual obligations have been satisfied with respect to that amount.