This Mishnah will talk about somebody who arrives in Yerushalayim with silver coins of Maaser Sheni (Second Tithe) and wants to change them for a smaller denomination of copper coins because he wants to buy food. The Mishnah says that somebody who exchanges a silver Sela coin of Maaser Sheni for copper coins in Yerushalayim is allowed to change the entire silver Sela for the small denomination of copper coins. That's permissible. It says no: he's not allowed to change the entire Sela for all those little Ma'ot (a smaller denomination of copper coins). That's not permissible. Rather, he has to take half of the Sela and change it into one Shekel (silver coin). Two Shekel equal one Sela. So 50% of it has to be turned into one full Shekel. The other 50% of the Sela that he has could be turned into smaller copper coins. So 50% has to go into a still-large denomination. Half of a Sela is a full Shekel. Then the other 50% he could turn into the smallest denomination, which is the copper coins.
He can't exchange the entire Sela for copper because, according to Bet Hillel, we're nervous that he's going to have all these copper coins and not spend all of them on food during that trip to Yerushalayim. Instead, he's going to hold some back and say, “You know what? When I come back next time, I'll have some coins, some Maaser Sheni coins to spend when I come here.” Silver coins oxidize. They turn green and would lose their value if left like that over time. So they would be worth less when he returns to Yerushalayim, and therefore he would have lost money that he could have used to purchase food with Maaser Sheni. So Bet Hillel says he can't do that.
Bet Hillel would also not suggest that he take the copper coins and switch them back into silver coins—a Sela or a Shekel—because at that point in time, there's always a fee when you go to the money changer. There is, of course, an exchange rate, but there's also a small fee tacked on to allow the money changer to make money. That would be an unnecessary waste of Maaser Sheni.
Another opinion: those who were talking in front of the Hachamim (rabbis) were debating this idea in front of the rabbis. Bartenura tells us that they were talking about specifically four different Hachamim: Shimon ben Azai, Shimon ben Zoma, Hanan ha-Mitzri, and Hananiah ben Hachinai. These four greats did not live into old age. They were Talmidim. They were students their entire lives. They held that he has to take three dinarin (dinar coins) worth of Sela. Three quarters of it have to remain in a Sela, in silver coins. Only a quarter of it, which is a dinar—one quarter of that original Sela, 25% of it—could be exchanged into copper coins. That's what they held. They were even more strict than Bet Hillel. A smaller denomination could only be exchanged into copper coins.
And if you look at the Hachamim, Rebbe Akiva says he has to take change for three dinarin worth of silver coins. The fourth dinar, he can't exchange that full fourth dinar for copper coins either. But only a quarter of that fourth could he change into copper coins. So, according to Rebbe Akiva, he could only exchange one sixteenth of the original Sela for copper coins. Rebbe Akiva says he has to take change for three dinarin worth of Sela in silver coins. That means he has to leave three quarters of it still in silver coins. And of that fourth dinar, he could take four units' worth in silver coins. That means only one twentieth of the original Sela could turn into copper coins. So you end up with a small, a very small amount of copper coins according to Rebbe Tarfon.
A last opinion: Shammai omer (Shammai says) he's not allowed to change silver coins for copper coins at all. What does he do? He takes the silver coin and deposits it with the storekeeper, and he has a credit. Over time, he'd eat away slowly at that credit until he reaches the point where he eats a full Sela. That's the opinion of Shammai. There's no changing whatsoever. Just eat away at your credit.