Subscribe to the Daily Daf Yomi Summary here.
The braisa states: If a document contained a provision to collect interest (in a forbidden fashion), we give him a fine. Not only is he unable to collect the interest, but he cannot even collect the principle. These are the words of Rabbi Meir. The Chachamim say: He can collect the principle, but not the interest.
Shulchan Aruch (C”M 52:1) rules according to the Chachamim that the principle may be collected.
The Ketzos Hachoshen (38:1) asks: If we hold that when one violates a negative precept, he cannot be liable to pay even if he does not receive lashes (chayvei malkos shogegin); then how can the principle be collected? Behold, the borrower is violating the prohibitions of “You shall not impose interest upon him,” and “You shall not give interest to your brother”? If so, why should the borrower be liable to pay? [This is a type of kim leih bid’rabbah minei (whenever someone is deserving of two punishments, he receives the one which is more severe)!?]
He answers based on one opinion that holds that the borrower does not transgress the prohibition of giving interest until he actually gives it. Accordingly, we can say that the principle of kim leih bid’rabbah minei does not apply here, for the liability to pay and the action which would theoretically cause him to be liable for lashes are not simultaneous. In such types of cases, he will not be exempt from paying.