Shevuot 40A

תדע, דקא תני סיפא: ״דינר זהב לי בידך״, ״אין לך בידי אלא דינר כסף וטריסית ופונדיון ופרוטה״⁠ – חייב, שהכל מין מטבע [אחת] הן. אי אמרת בשלמא שוה ⁠– משום הכי חייב; אלא אי אמרת דוקא ⁠– אמאי חייב?

The Talmud discusses the basis for Rav’s explanation. Know that the Mishnah is referring to claims of monetary value, as it teaches in the latter clause that if the claimant said: I have a gold dinar in your possession, and the defendant responded: You have only a silver dinar, or a tereisit, or a pundeyon, or a peruta in my possession, he is liable to take an oath, as they are all of one type; they are all coins. Granted, if you say that the claim was for the value of a gold dinar, it is due to that reason that he is liable to take an oath, as he admitted to a part of the claim. But if you say that the claim was specifically for a dinar of gold, why is he liable? The claim was for gold, and he admitted to owing silver or copper.

אמר רבי אלעזר: בשטענו בדינר מטבעות, וקא משמע לן דפרוטה בכלל מטבע איתא. דיקא נמי, דקתני: שהכל מין מטבע אחת.

The Talmud rejects this proof: R' Elazar says: This Mishnah is referring to a case where one claimed that the defendant owes him a dinar of coins, i.e., coins worth a dinar. The claim was for specific coins, and not a monetary value. Since the claim concerned coins and not a specific weight of metal, and all coins are used for commercial transactions, all types of coins are considered of the same type. And it teaches us that a peruta is included in the category of a coin. According to this explanation, the language of the Mishnah is also precise, as it teaches: The defendant is liable to take an oath, as they are all of one type [min]; they are all coins. In other words, even a peruta is a type of coin.