We are learning Mishnah 4 in the fifth chapter of Maseches Temurah. This Mishnah applies exactly the same principles that we used in the previous Mishnah, in order to teach that these rules hold true in the laws of temurah as well.
The structure of the Mishnah:
A person has two consecrated animals standing before him - an olah and a shelamim - and he is holding a third animal that is unconsecrated. He declares that the unconsecrated animal should serve as a substitute (temurah) for both of the consecrated animals. The halachic outcome depends on the wording he used and on his intent at the moment he spoke.
"Harei zo temuras olah u'temuras shelamim":
This is the case in which he said about the unconsecrated animal that it should be a substitute for the olah and a substitute for the shelamim. According to the first opinion in the Mishnah - "harei zo temuras olah" - it is a substitute for the olah alone. Since he said that it should be a substitute of two different kinds, Rabbi Meir applies the rule of "tafas lashon rishon": when a person says two things, and the second contradicts the first or cannot be reconciled with it, we follow the first statement - that is what he intended and that is what he said, and the second statement has no relevance at all. Therefore the temurah takes effect only for the first type he mentioned, namely the olah.
But Rabbi Yosi disagrees, just as he disagreed in the previous Mishnah. In his view the rule is not simply "tafas lashon rishon"; rather, "af b'gemar davar adam nitpas" - what he said second is also meaningful and takes effect, provided that both ideas were in his mind at the time he said the first one. These are the words of the Mishnah: "Amar Rabbi Yosi: im l'chach niskaven mitechilah, ho'il v'i efshar likro shnei sheimos k'echad - devarav kayamin" - if that was his intent from the outset, since it is impossible to pronounce two names at once, his words stand. That is, if at the moment he said "harei zo temuras olah u'temuras shelamim" both animals were in his mind, and he wished to make this single unconsecrated animal a substitute both for the olah and for the shelamim, and the only reason he did not say both together was that a person cannot pronounce two names simultaneously, since that is the nature of speech - then both of his statements are binding, and the unconsecrated animal becomes a substitute both for the olah and for the shelamim, a sort of half olah and half shelamim.
Beyond having transgressed the Torah's prohibition against making a temurah, he has now created a strange creature that is half and half. The ordinary law for an olah and a shelamim is that the substitute is itself offered on the Altar, as was explained in the third chapter, but here that is impossible: a shelamim is eaten while an olah is burned entirely, so what is to be done with this animal? Therefore the substitute animal is left to graze until it develops a blemish, and then he sells it and uses the proceeds to buy two offerings, one for an olah and one for a shelamim, each with half of the sale price. For example, if the lamb he sold was worth one hundred, he buys with the proceeds two lambs at fifty each, one for an olah and one for a shelamim.
The bottom line, then, is that we ask him what he intended; and if he answers that he intended both of them from the start, we believe him and both take effect.
The second case - he changed his mind:
The Mishnah continues: "V'im she'amar temuras olah, v'nimlach v'amar temuras shelamim - harei zo temuras olah" - if he said "a substitute for the olah," and then reconsidered and said "a substitute for the shelamim," it is a substitute for the olah. That is, if at the moment he made the first statement that alone was his intent, and only afterward did he change his mind and add "a substitute for the shelamim" in an attempt to retract - only the first takes effect, since when he made his first statement he intended one thing and not two. The halachah here follows Rabbi Yosi.
In summary: these are the rules that emerge from the Mishnah:
Ambiguous wording - "harei zo temuras olah u'temuras shelamim": we do not know whether he intended both statements as one continuous declaration, and therefore we ask him. If he said "I intended both," both take effect; if he said "I was trying to retract," his retraction is ineffective and only the first takes effect.
He did not repeat the word "temurah" - "harei zo temuras olah v'shelamim": here there is no doubt at all and no ambiguity. He did not retract, and he certainly intended both, and therefore his words take effect even without our asking him. In this case too the animal grazes until it develops a blemish, and he sells it and brings with the proceeds an olah and a shelamim.