(Chizkiyah): (The Mishnah in Maseches Bava Metzia 4:6 discusses a case when the weight of a Selah coin became deficient. The Mishnah asks, "How much deficiency in weight can there be before the owner violates the prohibition of Ona'ah (fraud)...? The Mishnah later says that 'one may redeem Ma'aser Sheni with it without fear'.) When he wishes to transfer the Kedushah from this coin onto produce in Yerushalayim, he only needs to redeem it according to its actual value. However, when he wishes to redeem the Kedusha of Ma'aser Sheni onto such a coin, he may use it as if it has the full value of a Selah.

Question: The Mishnah taught that he then selects the better of the two original Selahs and redeems the Kedushah from the copper coins back onto it. Why must he select the better one? Surely he should be permitted to redeem the inferior Selah and view as if it's a good Selah coin?

Answer (R. Yona): I say that the better coin was the Ma'aser Sheni.

Rebuttal: In any case, didn't it become Chulin when the Kedushah was removed from it?!

(R. Yona): Originally, he didn't intend to make it fully Chulin (when he transferred the Kedushah onto the Selah). And you should know that this is true, as the Mishnah taught that he must transfer the Kedushah back onto silver coins.

(R. Chaggai to R. Zeira that Menachem cited R. Yochanan): Any case where they said about Ma'aser Sheni of Demai that one may transfer the Kedushah; for certain Ma'aser Sheni one may transfer in the same way, but only in a case of need.

Question (our Mishnah): One may transfer the Kedushah from silver onto copper coins. This implies that one may not transfer from silver onto silver...?

(R. Ba bar Kohen to R. Yona that R. Acha cited from R. Yochanan): Any case where they said about Ma'aser Sheni of Demai that one may transfer the Kedushah; for certain Ma'aser Sheni, if a person transgressed and transferred, it is valid.

(R. Yudan ben Pazi/ R. Shimon bar Ba citing R. Yochanan): Any case where they said about Ma'aser Sheni of Demai that one may transfer the Kedushah; for certain Ma'aser Sheni, if a person transgressed and transferred, it is valid.

Baraisa (R. Elazar bei R. Shimon): Just as one may transfer from silver onto copper (in a case of need), so too one may transfer gold onto silver.

(Rebbi to R. Elazar bei R. Shimon): Why may one transfer silver onto copper? Since one may transfer silver onto gold. But may one transfer gold onto silver, if one may not transfer gold onto copper?

(R. Elazar bei R. Shimon to Rebbi): Yes, since one may transfer Ma'aser Sheni of gold onto copper (Perutah) coins in Yerushalayim.

From both of these opinions we see that one may transfer silver onto gold but one may not transfer gold onto copper.