הלכה על בבא בתרא 10:11

Asmachta is where a party agrees to pay a penalty in case he does not perform a specific task, where the penalty exceeds the damage he caused. The Mishnah (Bava Batra 10:5) presents the following situation:someone paid a part of his loan and sought an extension of the loan. The borrower gave the original promissory note to a third party and instructed him that if he would not repay the loan by the specified time, the third party may present the original promissory note to the lender, enabling the latter to collect the entire loan despite the fact that part of the loan was paid. Although Rabi Yosi validates this agreement and would instruct the third party to hand the note to the lender, the Halachah (Shulchan Aruch C.M. 207:12) follows the opinion of Rabi Yehudah who invalidates such an agreement.