Discharge of surety by variation in the principal contract...

Discharge of surety by variation in the principal contract operates prospectively: under Section 133 a surety is discharged only as to transactions subsequent to an unauthorised variance, while Section 139 applies only where the creditor's act or omission is inconsistent with the surety's rights and impairs the surety's eventual remedy. Applying these principles, the sureties guaranteed liability up to the originally sanctioned cash credit limit and are discharged only for excess overdrafts occurring thereafter; there was no impairment of remedy, so Section 139 does not apply and liability remains limited to the sanctioned amount with interest.