Maaser Sheni, Chapter Two, Mishnah Eight
This mishnah continues to deal with what types of maaser sheni coins may be exchanged for which others.
1) One who changes copper coins of second tithe for a sela:
a) Bet Shammai says: he may change copper coins for a whole sela.
b) But Bet Hillel says: a shekel of silver and a shekels worth of copper coins [can be exchanged for the sela].
2) Rabbi Meir says: one may not exchange silver and produce for silver.
a) But the sages allow it.
Section one: The person in this mishnah has some copper coins that he used to redeem second tithe produce and he wants to exchange the copper coins for a silver sela, so that he doesnt have to carry all of the copper to Jerusalem. According to Bet Shammai, if he has enough copper coins to equal an entire silver sela, he can exchange them. However, if he only has enough coins for half of a sela, and for the other half he has a silver shekel (worth half of a sela) of maaser sheni money, he cannot exchange the copper and the silver shekel for the silver sela because silver maaser sheni money cannot be exchanged for other silver.
Bet Hillel says that one can exchange copper coins and a silver shekel for a silver sela. Even though in general it is forbidden to exchange silver maaser sheni coins for other silver maaser sheni coins in this case it is permitted because he is exchanging copper coins for half of the sela.
Section two: If someone has half a selas worth of produce, and half a selas worth of silver coins, according to Rabbi Meir he may not exchange all of them for a silver sela. This is similar to Bet Shammai in section one who held that one cannot exchange silver coins for another larger silver coin, even if some of the larger coin is being exchanged for copper. So too, Rabbi Meir holds that one may not exchange silver coins for larger silver coin, even if part of the exchange is being done with produce, which is usually a permitted exchange.
The other sages stick closer to Bet Hillel and rule permissively. Just as one can exchange copper and silver for silver, so too one can exchange produce and silver for silver. As long as half of the exchange is done in a normally permitted manner (silver for produce or copper) the rest may be done in a normally forbidden manner (silver for silver).