In Parshas Behar the Torah teaches some of the halachos of ribbis (charging interest on a loan). The Torah says that one should not take ribbis from his fellow and should fear God, “v’chei achicha imach – and your brother shall live with you.” The Gemara derives from the Torah’s wording that a person who charges ribbis illegally, and then collects it, must return the money.
There is a question regarding returning interest money. When one steals, the money or item stolen must be returned. The item is still considered the original owner’s. When the thief returns the item, he is in fact returning an item that is already the owner’s. It is for this reason that the thief can return the item even if it has gone down in value. Since it is the original owner’s item, he is only obligated to return it as is. (This rule does not apply if the item has undergone physical or other changes, for then we no longer view it as the same item, and the value of the item at the time of its theft is owed to the owner.)
If a person steals an item and then dies, his children must return the stolen item. However, the Gemara (Baba Kama 112a) says that the obligation to return ribbis lies only with the one who charged the ribbis. If a person inherits money that his father received as a ribbis payment, he does not have to return the money.
The Ritva (to Kiddushin 6b) derives from this halacha that when ribbis is owed, it is a real debt, and when it is paid to the lender the lender owns the money. This is in contrast to the general rule regarding money obtained illegally. Although the Torah forbids charging and collecting ribbis, when two people arrange a deal that involves ribbis, the ribbis is actually owed. Therefore, if and when it is paid, it belongs to the recipient – for it was actually owed to him. Nonetheless, he is obligated to return it. (He is also obligated not to collect in the first place.) Until he returns it, though, the money belongs to him. Thus, his heirs can keep the money, as it actually belonged to their father. In this case, the obligation to return the money only devolves on the father; his heirs do not have to return the money.
Based on this halacha, the Ritva argues that if a person took money that he received for ribbis and married a woman with it, the kiddushin is valid. If a woman owes a man money and he forgives her that debt on condition that she be married to him, the kiddushin is valid. This is because forgiving a debt is considered something of value. The Bach (Even Ha’ezer 28) writes that the halacha would be the same if a woman owed a man ribbis and he forgave her the debt. Since the ribbis money is actually owed to him, his forgiveness of the ribbis debt is considered something of value – and that results in a valid kiddushin.
Tosfos (Baba Metzia 62a) suggests that one who receives lashes for collecting ribbis is still obligated to return the ribbis.
The Pnei Yehoshua writes that one who receives lashes for collecting ribbis should be exempt from repaying the ribbis because we should apply the rule of kim lei b’deraba minei – one receives only the more stringent punishment of two punishments one incurs. He shouldn’t have to both receive lashes and repay the money. Why, then, must he?
Rav Chaim Soloveitchik suggests that the obligation to return ribbis is different from the obligation to return a stolen item. A stolen item does not fully belong to the thief; as we have explained, it still belongs to the original owner. A ribbis payment, on the other hand, fully belongs to the lender. Therefore, we cannot compare a person’s obligation to return ribbis to a general obligation to return money. The obligation to return ribbis is a separate mitzvah that the Torah obligated the lender to perform, which we derive from “v’chei achicha imach.” Since the payment is not a punishment for transgressing the aveirah of charging ribbis – but rather a separate mitzvah – he receive lashes and must return the money.