I asked a friend who was learning in E. Y. and returning home for Succos, to buy me an esrog and bring it with him when he comes for Yom Tov. When he brought me the esrog he told me that he spent the equivalent of ninety dollars. I didn’t have ninety dollars in my pocket so I gave him a hundred dollar bill and told him to keep the change. A few days later he brought me a ten dollar bill and told me that he never intended to keep the change because it would be ribbis. I was quite taken aback because I didn’t intend to give interest on a loan and didn’t even realize that I borrowed any money. Is he correct?
Your question really is two questions. The first question is whether your friend lent you money and the second question is whether even if he did lend you money whether paying a little more would be prohibited as ribbis.
In order to determine the answer to the first question it is necessary to analyze what happened when your friend bought you the esrog. The question is whether he bought the esrog for himself or for you. If he bought it for himself then what happened is that he bought himself an esrog in Israel and he sold it to you when he brought it to you, in which case you never borrowed money from him. If one follows this approach your friend was fully responsible for the esrog until you accepted it from him since it was his esrog.
However, if the esrog belonged to you as soon as he purchased it, then he would only have had the status of a shomeir-watchman during the time between the purchase and your acceptance of the esrog. Since he was not paid for watching the esrog he would have the limited liability of an unpaid watchman-shomer chinom, who is only liable for negligence.
We can prove that people’s understanding, and that is also the understanding of the SA, is that when your friend paid for the esrog he acted as your agent and you immediately became the owner.
We can prove that is what people think from two frequent situations. One situation that occurs every year is that U. S. Customs confiscates esrogim. If you were the owner of the esrog you would have to suffer the loss but if he was the owner he would have to bear the loss. Generally, the one who ordered the esrog pays for the esrog even when it is confiscated, which indicates that people view the final user of the esrog as being the owner of the esrog even when it is in transit.
A second situation that sheds light on people’s understanding occurs when the recipient is not happy with the esrog that he receives. If he is already the owner, he cannot refuse to accept the esrog. But if the one who brought it from Eretz Yisroel is the owner, the recipient can refuse to accept the esrog. Since people never refuse to accept, it indicates that their understanding is that they became the owner immediately, at the time of purchase.
This is the explicit ruling of the SA (CM 183, 4) as well. The SA goes so far as to rule that even if a messenger, who was asked to buy a specific object with his own money claims after he bought the object that at the time of purchase he intended to buy it for himself, he is not believed. This is derived (Bach 183) from a ruling of the Gemoro (BM 10A ruled in CM 269) that if a person asks someone to pick up an ownerless object on his behalf, the one who picked it up is not believed if he claims afterwards that when he picked it up he meant to keep it for himself. The reason (Sema 183, 9) is that we assume that people want to be reliable and to do as they were asked to do. Therefore, if the messenger failed to inform the one who requested him to buy the object prior to its purchase that he was purchasing it for himself, we assume that he meant to buy it for the one who requested the purchase and he is not even believed if he claims otherwise.
Since your messenger used his own money to buy the esrog on your behalf he effectively loaned you the money that he used to pay for the esrog. The Shach (183, 2) and Nesivos (183, 2) explain that this is how you became the owner of the esrog when the messenger actually paid the seller with his own money. They explain that since the messenger lent you the money, he really paid for the esrog with your money and the intent of the seller (as it always is) was to transfer ownership to the one whose money paid for the esrog. (This is the explanation of the Gemara in BK 102B.) You received the money by the seller’s acceptance of the money on your behalf.
This is similar to the way the Gemara in Kiddushim (7A) explains how a man can wed a woman by giving money to a third party at the request of the woman. There is a very difficult responsa of Iggros Moshe (CM 1, 67) where he disagrees with the SA, Ramo, Shach and Nesivos, and claims one can’t extend a loan in this manner but his argument is very difficult to understand. Perhaps, even he only disagrees if the messenger was paid for his services, in which case his opinion would not be germane to this discussion. The Kisei Mishpat (87) also brings Rishonim who disagree with the Iggros Moshe.
Having established that you actually borrowed money from your messenger when he paid for your esrog, we must address the second issue, namely whether you would have violated the prohibition of ribbis if you had paid the messenger a hundred dollars, when he only paid ninety dollars for your esrog.
When one who borrows money commits himself at the time the loan was made to pay more than the amount he borrowed, both the lender and the borrower violate the Torah’s injunction against charging interest when the borrower actually returns more than the amount he borrowed. However, in your case there was no commitment from you, the borrower, to return more than the amount you borrowed. And in fact your messenger never asked for the extra money. Nevertheless, the SA (YD 160, 4) rules that the Rabbonon forbade a borrower to give at the time he repays his loan, more than the amount that he borrowed, even if the borrower does not state that he added funds in appreciation for the loan, since by giving extra at the time of repayment it appears that the borrower is paying interest.
If you had simply given your friend a hundred dollars and not mentioned anything about the extra money you were giving, many Acharonim (Prisho, Bach, Taz) rule that if it was clear to your friend that you didn’t make a mistake, he could keep the entire hundred dollars. However, others (Shach) say even then that it is forbidden. In your case since you pointed out that there was extra money, even the lenient opinion would rule that there is an issue of ribbis.
We should note that since the prohibition against giving extra at the time of repayment is due to appearances, the prohibition applies even if the one who gave extra otherwise gives presents to the lender. Similarly, it applies even if the intent of the borrower to give extra is not related to the loan. The Minchas Yitzchok (9, 88) rules that even if one does not have exact change he can’t pay extra and tell the borrower to keep the change. What the borrower can do is tell the lender to give the change to tsedoko. The Nesivos Sholom rules that if someone borrowed half of a bottle of soda he may return a whole bottle since it is clear that he is doing so because it’s embarrassing to return half a bottle.
In conclusion: Your friend is correct that there was a problem of ribis with what you did and he acted properly by not intending to accept the extra ten dollars. We should note two points. One is that had your friend not been so knowledgeable and he had intended to accept the hundred dollars, some (Chavos Da’as) rule that he would not need to return the extra money later on since the status of the extra money is that of ribis me’ucheres-ribbis paid after the loan was repaid even though it was given at the time of repayment. However, many disagree with this opinion. The second point is that you could have given extra if you would have stated at the time that you were giving the extra ten dollars that it is payment for your friend’s efforts in buying you the esrog.