Bava Metzia 44B

דחריף ⁠– הוי טבעא. דהבא דלא חריף ⁠– הוי פירא, וקני ליה פירא לטבעא.

which circulate, in the sense that they are universally accepted by merchants, in contrast to gold coins, which merchants are less willing to accept as payment for inexpensive items, are currency; gold coins, which do not circulate, are a commodity. And the principle is: When one party takes possession of the commodity, the other party acquires the currency.

אמר רב אשי: כילדותיה מסתברא, מדקתני: ״הנחשת קונה את הכסף״.

Rav Ashi said: It is reasonable to teach the halakha in accordance with that which he taught in his youth. This is from the fact that the tanna teaches later in the Mishnah: When one party takes possession of the copper coins, the other party acquires the silver coins.

אי אמרת בשלמא כספא לגבי דהבא פירא הוי, היינו דקא תני: ״הנחשת קונה את הכסף״, דאף על פי דלגבי דהבא פירא הויא, לגבי נחשת טבעא הוי. אלא אי אמרת כספא לגבי דהבא טבעא הוי, השתא לגבי דהבא דחשיב מיניה, אמרת טבעא הוי, לגבי נחשת, דאיהו חשיב ואיהו חריף ⁠– מבעיא?

Rav Ashi explains: Granted, if you say that the silver coins relative to the gold coins are a commodity, that is the reason that the tanna teaches: When one party takes possession of the copper coins, the other party acquires the silver coins, as, even though relative to the gold coins, the silver coins are a commodity, the tanna teaches that relative to copper coins, they are currency. But if you say that the silver coins relative to the gold coins are currency the subsequent ruling is self-evident, as now, relative to the gold coins, which are more valuable than the silver coins, you say that silver coins are currency, then relative to copper coins, as the silver coins are more valuable than the copper coins and they also circulate more easily, is it necessary for the Mishnah to state that the silver coins are currency and the copper coins are a commodity?