We cannot respond to the question’s “Choshen Mishpat” elements (i.e., the various monetary claims of the sides) without hearing both sides. We will focus only on the “Yoreh Deah” (i.e., ritual, religious halachot) issue of ribbit[2] on the loan, which depends on the possibilities of what the exact financial arrangements and legal constructs between the parties were. We will relate to the major possibilities.
It sounds like Levi accepted the responsibility to repay a 15,400 NIS loan. If Shimon was simply an agent following instructions, he has no more obligation than that of a simple conduit; apparently, he merely accepted upon himself to take care of the currency transfers. Therefore, he returns to Reuven only the amount of dollars he can get from the shekel sum Levi paid him.
If Shimon in fact accepted responsibility for payment, there were either two separate loans (one loan from Reuven to Shimon, which then enabled a second loan from Shimon to Levi) or Shimon was an arev (guarantor). We will now analyze each possibility.
If there were two loans, Shimon would ostensibly need to return the $4,500 loan he received from Reuven. Is it permitted for an Israeli to return the same number of dollars as he received if the value of those dollars has gone up, or is that a problem of ribbit? The rule is that it is Rabbinically forbidden to lend a specific amount of objects that, at the completion of the loan period, are to be replaced and given to the lender in the same amount of the same type of object. This is because this commitment might require the borrower to return more value than he received, if the object went up in value over the course of the loan.[3] However, that is true of commodities, whereas currency is viewed halachically as a constant (even if its true value, as compared to commodities and other currencies, changes). Accordingly, one might conclude that a dollar loan is permitted, and Shimon would therefore be obligated to return the same $4,500 he received, even if the sum’s shekel equivalent increased over the time.[4] On the other hand, outside the country where a currency is the legal tender, it is considered a commodity. Although relatively recent poskim have ruled that dollars have a special status in Israel, because of their centrality in many aspects of day-to-day finance,[5] the situation of centrality that this ruling was based on no longer exists; dollars are not sufficiently used in today’s Israel to be considered a currency here. Nonetheless, when an American transfers dollars from America and wants that same amount of dollars returned to him, dollars are considered the relevant currency, and there is no problem of ribbit for their return.[6] Thus, if these were indeed the circumstances, Shimon may and should return $4,500 to Reuven.
If there was only one loan (from Reuven to Levi) and Shimon is merely an arev, we must analyze which of the three types of arev he was: 1) A simple arev – one who pays only if the borrower defaults;[7] 2) An arev kablan – in which case the lender may choose to take payment either from the borrower or the arev;[8] 3) An arev sh’lof dotz – in which case the lender receives payment specifically from the arev.[9]
The gemara[10] states that if a non-Jew lends to a Jew a loan with interest, and the lender takes payment with interest from the loan’s Jewish arev, the borrower may not reimburse the arev for the interest. The gemara explains that since the non-Jewish practice is to go directly to the arev, it is considered as if the Jewish arev borrowed from the non-Jew and then lent the principal to the Jewish borrower. Therefore, the arev must not take back more money from the borrower than he gave him. There is a machloket among Rishonim, and there are two opinions in the Shulchan Aruch,[11] regarding if this prohibition is true only for an arev shlof dotz or even for an arev kablan.[12]
While that case is different from ours, the analysis of how to view the financial relationships between the three parties in a loan with various types of guarantors should be the same. If Shimon is a simple arev or, according to some, even if he is an arev kablan, the loan is treated as one between Reuven and Levi. Thus, if Levi may simply pay the same amount of NIS that he received, Shimon is governed by the same rules. If so, neither Levi nor Shimon may give $4,500, unless one of the leniencies of this Rabbinic form of ribbit applies.[13]
If Reuven indicated that he was giving a dollar loan, he deserves to receive $4,500, and Levi is obligated to pay however much NIS is necessary for that to happen. Shimon is allowed to add from his own pocket to the money Levi gave him so as to return the full dollar amount to Reuven. This could occur if, for example, he wishes to help Levi and/or if he blames himself for having failed to relay to Levi that Reuven had indicated that the loan was to be repaid in dollars. It is certainly not a violation of ribbit if Shimon does not demand the difference from Levi.
If Shimon is an arev shlof dotz, it is likely that the loan was in dollars. In that case, Shimon may and should pay $4,500.[14] Whether and under what circumstances he would be permitted to ask for compensation from Levi for the added shekels needed to pay that amount of dollars is too complicated to discuss in this forum.
Fine nuances can affect the way to view these matters. It is laudable (not required) for Reuven to waive the possible right to full payment, whether he does so to avoid the possibility of a violation of ribbit or, especially, if he does so in order to strive for higher moral ground. (His dollar loss can count as tzedaka if the person who ultimately benefits from not being charged the difference, whether it be Shimon or Levi, is poor.[15])