Bava Metzia 45A

אסור ללוות דינר בדינר.

It is prohibited for one to borrow a dinar and repay the loan with a dinar, because if the value of the dinar changes in the interim, both the borrower and the lender will have violated the prohibition against interest.

דינר דמאי? אילימא דינר של כסף בדינר של כסף, לגבי נפשיה מי איכא למאן דאמר לאו טבעא הוי? אלא פשיטא דינר של זהב בדינר של זהב, ולמאן? אי לבית הלל ⁠– הא אמרי טבעא הוי. אלא לאו לבית שמאי, ושמע מינה רבי יוחנן הוא דאמר: אין מחללין!

The Talmud elaborates: The reference is to a dinar of what type of metal? If we say the reference is to one who borrowed a silver dinar and repaid the loan with a silver dinar, is there anyone who says that silver relative to itself is not currency? Rather, it is obvious that the reference is to one who borrowed a gold dinar and repaid the loan with a gold dinar. The Talmud continues its analysis: And in accordance with whose opinion does R' Yoḥanan state this halakha? If it is in accordance with the opinion of Beit Hillel, don’t they say that a gold dinar is currency? Rather, isn’t it in accordance with the opinion of Beit Shammai? And learn from it that it is R' Yoḥanan who said that according to Beit Shammai we do not desacralize produce with gold dinars, as he holds that they are not considered currency.

לא, לעולם אימא לך רבי יוחנן הוא דאמר מחללין, ושאני הלואה כיון דלענין מקח וממכר שויוהו רבנן כי פירא, דאמרינן: איהו ניהו דאוקיר וזיל ⁠– לגבי הלואה נמי פירא הוי.

The Talmud rejects this proof: No, actually I will say to you that it is R' Yoḥanan who said that even according to Beit Shammai we desacralize produce with gold dinars. And the halakha of a loan is different from the halakha of desacralizing second tithe, as with regard to buying and selling, the rabbis deemed the legal status of a gold dinar like that of a commodity. As we say: It is the gold that appreciates and depreciates in value, in accordance with the halakha in the Mishnah: When one party takes possession of the gold coins, the other party acquires the silver coins. With regard to a loan as well, the legal status of a gold dinar is like that of a commodity, and therefore there is concern that they may violate the prohibition against interest.