Before us is Mishnah 6, the last in chapter 5 of Maseches Temurah. The previous Mishnah dealt with the expressions that are effective in creating a temurah, and here we will deal with the correct wording of the declaration.
The need to identify a specific animal:
The foundation of this Mishnah is that one who declares a temurah, meaning that an unconsecrated animal should take the place of a consecrated animal, must have in mind a specific, existing consecrated animal that the unconsecrated animal will replace. The Mishnah presents three formulations:
"Harei zu tachas chatas" or "Harei zu tachas olah" - he has said nothing. Such a statement has no halachic meaning and does not take effect at all, since he did not identify a particular chatas or olah from which the sanctity would pass to the unconsecrated animal.
"Tachas chatas zu" or "tachas olah zu" - when a specific animal that was consecrated as a chatas or an olah stands before him, with an unconsecrated animal alongside it, and he says that the unconsecrated animal should take its place - his words certainly take effect, and there is no real novelty in this.
"Tachas chatas v'sachas olah she'yesh li b'soch habayis" - here lies the novelty: even though the animal is not explicitly before him, since he has one particular chatas or one particular olah in his house and that is what he has in mind, it turns out that he has identified a specific animal that will be the source of the sanctity passing to the unconsecrated animal - and therefore his words stand and take effect.
Consecrating animals that are unfit:
From here the Mishnah opens a new topic. It is no longer discussing temurah, but rather an initial consecration - the consecration of animals that are unfit to be brought as an offering:
"Amar al beheimah teme'ah v'al ba'alas mum harei elu olah" - one who says about an animal that is not fit for an offering, such as a camel, or about a fit animal that developed a permanent blemish, such as a cow whose leg was cut off, that these are an olah - he has said nothing. His statement has no halachic force whatsoever, since there is no such thing as an olah made of a camel or an olah made of a blemished animal.
"Harei elu l'olah" - with the addition of a single letter lamed. This fine distinction changes the meaning of his words fundamentally: he does not intend to apply to the camel the inherent sanctity of an offering, but rather to designate it with monetary sanctity, for its value, in order to purchase an olah with it. Just as a person can say that this money is designated for the purchase of an olah, or that this barrel of wine is consecrated so that an offering will be bought with its value, so too he can say that the camel is designated, for its value, toward the purchase of an olah.
Therefore the Mishnah concludes: "Yimachru v'yavi b'dmeihen olah" - the camel or the blemished animal is sold, the sanctity of the value of an olah takes effect upon the proceeds of the sale, and with them a fit olah offering is purchased and brought.