Chapter 2, Mishnah 8. This mishnah deals with someone who is outside Jerusalem holding copper coins of Maaser Sheni, and who wishes to convert them into silver coins - in order to lighten the load he must carry and bring it up to Jerusalem. The mishnah discusses exactly how this is to be done.
Converting the copper coins into silver currency:
One opinion: He may take all of the copper coins and transfer their sanctity onto a sela - a large silver coin. In this way he rids himself of all the copper coins in his possession, and he is not required to keep any copper coins at all with him on his way to Jerusalem. (True, he may be left with a certain amount of change, but theoretically, if he had the exact sum, he could convert all of it.)
The dissenting opinion says: He may convert only half - a silver shekel, which is generally half a sela, should be made into actual silver, while the other half remains in coins, which are generally copper.
The reason: upon arriving in Jerusalem, a person will immediately need small change, the copper coins, since the large bills are of no use to him. It is comparable to the difference between a hundred dollar bill and a twenty: to actually purchase things you need the smaller bills - and those are the copper coins.
And if all those going up were to turn to the money changers to break the silver selaim in their possession and receive copper coins in exchange, the value of the copper coins would rise: once many people want them a great demand is created, and when there is great demand the value goes up, and the person ends up losing. Therefore the dissenting opinion says that he should bring some copper coins along with him, and that is the reason for dividing it in half.
Redeeming silver and produce onto silver:
In the previous mishnah we mentioned that one does not redeem silver onto other silver: this is not the normal manner of redemption, which is between different types - copper onto silver or silver onto copper, but not from silver to silver.
The question our mishnah raises: what is the law when a little silver and a little produce are combined together, and he intends to transfer both of them onto a larger silver coin? For example, he has half a dinar of silver of Maaser Sheni and half a dinar's worth of produce of Maaser Sheni, and he wishes to redeem both of them onto a silver coin of a full dinar. Is he permitted to do so?
Rabbi Meir says: "Ein mechallelin kesef ufeiros al kesef" - one who has half a dinar of silver of Maaser Sheni and half a dinar's worth of produce of Maaser Sheni, and wishes to redeem both of them together onto a dinar's worth of non-sacred silver, that is, to transfer the sanctity of both halves onto that one whole coin - he may not. This is considered redeeming silver onto silver, even though the silver here is joined together with produce.
Vachachamim matirin - the Sages permit it, because there is a need here: the produce is worth only half a dinar, and he wants to have a whole dinar of silver in hand. Therefore he is permitted to add to it a silver coin of half a dinar and redeem both of them together, so that he will have a full dinar of silver.
However, even the Sages agree that if he has produce worth a full dinar, he may not add to it a full dinar of silver and transfer both of them onto a shekel (since two dinars equal a shekel). The reason: he already has produce worth a whole coin, and he is able to redeem the produce worth a dinar onto a dinar of silver, and he is under no obligation to go all the way up to a shekel. There is therefore no justification here for redeeming a dinar of silver onto a shekel of silver.
In summary: in this mishnah we learned two matters: converting the copper coins of Maaser Sheni into silver for the journey up to Jerusalem - whether the entire sela may be made into silver, or half silver and half coins, because of the immediate need for small coins in Jerusalem and the concern that their value will rise; and the law of redeeming silver and produce together onto silver - that Rabbi Meir forbids it, while the Sages permit it where there is a need, as long as the produce itself is not worth a whole coin.