The debtor cannot exempt himself from the performance of the obligation by paying the penalty, save in the case where this right has been expressly reserved for him. Neither can the creditor demand the fulfillment of the obligation and the satisfaction of the penalty at the same time, unless this right has been clearly granted him. However, if after the creditor has decided to require the fulfillment of the obligation, the performance thereof should become impossible without his fault, the penalty may be enforced. (1153a)