194
Answer: We cannot respond to the question’s "Choshen Mishpat" element without hearing both sides. We will focus on the Yoreh Deah question of ribbit on the loan, which depends on the possibilities of what the arrangements between the parties were (we will relate to the major ones).
It sounds like Levi accepted the responsibility to repay a 15,400 shekel loan. If Shimon was but an agent who followed expectations, he returns only the dollar equivalent of what Levi paid. If Shimon accepted responsibility for payment, he was either an arev (guarantor) or there were two separate loans (Reuven-Shimon, enabling Shimon-Levi).
If there were two loans, Shimon must return his $4,500 loan from Reuven. Is this permitted when the value went up? The rule is that it is Rabbinically forbidden to lend an object so that it be replaced by an object of the same type, due to the chance that its value will rise over the course of the loan (i.e., he will return more value than he borrowed). However, since we halachically view currency as a constant (even if its value, as compared to commodities and other currencies, changes), $4,500 can be returned even if its shekel equivalent increased. Admittedly, currencies are considered commodities outside their country, and the leniency that dollars have a special status in Israel no longer applies (see Igrot Moshe, Yoreh Deah III:37). However, when an American in America transfers dollars and wants dollars returned, one cannot say that this is not currency (see ibid.). Thus, under these circumstances, Shimon can return $4,500 to Reuven.
If there is one loan and Shimon is an arev, we should consider the three types of arev. 1) Simple arev – he pays only if the borrower defaults; 2) Arev kablan – the lender can choose to take payment either from the borrower or the arev; 3) Arev shlof dotz – the lender receives payment specifically from the arev.

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If Shimon is an arev shlof dotz, it is likely the loan was in dollars, in which case, Shimon may and should pay $4,500 (see Netivot Shalom, p. 349). He can decide, based on his discussions with Levi, whether to ask for reimbursement from him.
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, for halachic safety but especially for higher moral ground. (His dollar loss counts as tzedaka).

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