Maaser Sheni, Chapter 1, Mishnah 2. The Mishnah opens with the laws of maaser behemah (the animal tithe), continues with the laws of the bechor (firstborn animal), and concludes with the laws of redeeming Maaser Sheni.
Maaser Behemah:
Maaser behemah applied to someone who owned animals that gave birth: from the animals born in that year he had to separate a tithe, and one out of every ten animals was brought as an offering in the Beis HaMikdash.
"Maaser behemah, ein mochrin oso tamim chai" - there is a prohibition against selling maaser behemah, derived from the verse, and one may not sell it while it is unblemished - that is, free of any mum (blemish) - and still alive. It is worth noting that it may not be sold after being slaughtered either, and the Mishnah specified "unblemished and alive" only because it needs that wording for the case of the bechor in the latter part of the Mishnah.
"V'lo baal mum chai v'shachut" - even once it has developed a blemish, whether alive or slaughtered, it may not be sold. It emerges that maaser behemah may never be sold, in any situation whatsoever: unblemished or blemished, alive or slaughtered.
"V'ein mekadshin bo es ha'ishah" - one may also not betroth a woman with it, since betrothal is a form of sale, even though the animal belongs to him.
The Bechor:
The bechor is given to a kohen, and when the Beis HaMikdash stands the kohen brings it as an offering. Nowadays the kohen must wait until it develops a blemish, and then he slaughters it and eats it.
"HaBechor mochrin oso tamim chai" - it may be sold while it is unblemished and alive.
"U'baal mum chai v'shachut" - once it has developed a blemish, he may sell it whether alive or slaughtered, since from the moment it becomes blemished it belongs to the kohen.
The only situation in which a bechor may not be sold is when it is unblemished and slaughtered - free of any blemish and already slaughtered.
"U'mekadshin bo es ha'ishah" - as stated in this clause, for the reason that it is his own money.
The reason for this: if one slaughtered a bechor before it developed a blemish - and before it develops a blemish it is forbidden to slaughter it - the bechor requires burial and is forbidden to derive any benefit from it, and therefore it may not be sold. From here we see that the Mishnah is dealing with a time when the Beis HaMikdash does not stand, for in the time of the Beis HaMikdash even an unblemished live animal could not be sold: an unblemished, living bechor must be brought by the kohen as an offering, and he has no personal ownership of it; rather, it belongs to the Beis HaMikdash. The Mishnah therefore cannot be speaking about the time of the Beis HaMikdash, but about the period after the destruction - and then an unblemished live animal may be sold, an unblemished slaughtered one may not be sold because it requires burial, and a blemished one may be sold whether alive or slaughtered, since from the moment it becomes blemished it belongs to the kohen.
Redeeming Maaser Sheni:
Having concluded the laws of maaser behemah and the bechor, the Mishnah teaches a further law concerning Maaser Sheni and redeeming it onto coins:
"Ein mechalelin maaser sheni al ha'asimon" - one may not redeem Maaser Sheni with a coin that bears no form or stamp, but is merely a lump of metal. Redemption requires a coin that has a form on it, and this law is derived from the verse.
"V'lo al hamatbeia she'eino yotzei" - one may not redeem with a coin that is not accepted by merchants and has no currency, since it cannot be used as money. For the verse instructs one to use the money to buy in Yerushalayim whatever he desires and to eat it, and money that is not accepted by merchants and cannot be spent buys nothing.
"V'lo al hamaos she'einan birshuso" - money that is not actually in his possession and under his control.
For example: a purse of coins that fell into the sea, and a diver would have to be hired to retrieve it - it is not considered to be in his possession, and therefore a person cannot redeem onto it and say: I have a purse of coins at the bottom of the sea, and I wish the maaser to be redeemed onto it. Such a redemption is ineffective.