Montana Title 28. Contracts and Other Obligations § 28-11-211. When guarantor exonerated

(1) A guarantor is exonerated, except so far as the guarantor may be indemnified by the principal, if by any act of the creditor not authorized by the original guaranty instrument or without the separate consent of the guarantor, the original obligation of the principal is altered in any respect or the remedies or rights of the creditor against the principal in respect to the original obligation are in any way impaired or suspended.

(2) A promise by a creditor that for any cause is void or voidable by the creditor at the creditor's option does not alter the obligation or suspend or impair the remedy within the meaning of subsection (1).