136. Surety not discharged when agreement made with third person to give time to principal debtor. Where a contract to give time to the principal debtor is made by the creditor with a third person, and not with the principal debtor, the surety is not discharged. Illustration C, the holder of an overdue bill of exchange drawn by A as surety for B, and accepted by B, contracts with M to give time to B. A is not discharged.