Maaser Sheni, Chapter Two, Mishnah Six. This mishnah presents us with a principle: someone who holds silver coins of Maaser Sheni may not redeem them onto other coins. Redemption of Maaser Sheni coins is done onto food, not onto coins. There are, however, cases, such as the one before us, in which one is required to redeem a coin onto a coin, and then it must be done by way of changing the metal from one type to another: silver coins are transferred onto copper coins and the reverse, since this is considered a valid manner of transfer.
The reason for this: exchanging an item for an item on the same level is no more than a sideways move, and it does not constitute a manner of redemption; only a change from one type to another type is considered a manner of transfer, and that is the way redemption works.
The case in the mishnah:
"Sela shel maaser sheni v'shel chulin shenisarvu" - a sela is a silver coin. If a sela of Maaser Sheni and a sela of chulin (ordinary produce) became mixed together, and one does not know which is which, we do not take the simple route and say: I will choose one of the two and declare that if it is Maaser Sheni, fine, and if it is not Maaser Sheni, let the one that is Maaser Sheni be transferred onto it. Rather, "meivi b'sela maos" - he brings copper coins worth a sela. Since copper is of lesser value, many coins are needed in order to equal the value of the silver sela of Maaser Sheni.
The stages of the redemption:
"V'omer: sela shel maaser sheni b'chol makom shehi mechulelet hi al hamaos ha'elu" - the sanctity of the maaser passes from the sela, whichever of the two coins it may be, onto the copper coins. Now both silver coins are chulin, and neither of them is Maaser Sheni.
"U'vorer et hayafeh shebahen" - he selects the nicer of the two silver coins, since we want the Maaser Sheni to rest specifically on the nicer of the coins.
"U'mechalel aleha" - he transfers the copper coins that carry the sanctity of Maaser Sheni back onto that nice silver coin, and it is the official Maaser Sheni coin.
"Mechalel kesef al nechoshet midochko":
"Mipnei she'amru" - the reason for this procedure: "mechalel kesef al nechoshet midochko". In such a pressed situation, where he has no other option, he may transfer silver onto copper and redeem the silver onto the copper. And even this is not so that it should remain that way, but because he is going to transfer it again, redeeming the sanctity back onto silver. It is a temporary measure: we require him to redeem onto the copper, but the redemption does not remain that way permanently, and we want the sanctity to return to the silver.
In summary: in this mishnah we learned that Maaser Sheni coins are not redeemed onto other coins except by way of changing the metal. In the case of a sela of Maaser Sheni that became mixed with a sela of chulin: he transfers the sanctity onto copper coins worth a sela, selects the nicer of the two silver coins and transfers the sanctity back onto it - for transferring silver onto copper is done only under pressure, and as an intermediate stage whose end is a return to silver.