Discharge of surety by unilateral contract variation: non-consensual changes free surety from subsequent liability under the guarantee. A surety is discharged as to transactions subsequent to any variation in the contract between the principal debtor and creditor made without the surety's consent. Material changes to the debtor's duties, remuneration, payment arrangements, or timing that alter the nature or extent of the obligations secured by the guarantee relieve the surety of future responsibility unless the surety has consented to the variation.