Commentary for Bekhorot 2:3
כֹּל שֶׁקָּדַם הֶקְדֵּשָׁן אֶת מוּמָן, אוֹ מוּם עוֹבֵר לְהֶקְדֵּשָׁן, וּלְאַחַר מִכָּאן נוֹלָד לָהֶם מוּם קָבוּעַ (וְנִפְדּוּ), פְּטוּרִין מִן הַבְּכוֹרָה וּמִן הַמַּתָּנוֹת, וְאֵינָן יוֹצְאִין לְחֻלִּין לִגָּזֵז וּלְהֵעָבֵד, וּוְלָדָן וַחֲלָבָן אָסוּר לְאַחַר פִּדְיוֹנָן, וְהַשּׁוֹחֲטָן בַּחוּץ חַיָּב, וְעוֹשִׂין תְּמוּרָה, וְאִם מֵתוּ יִקָּבֵרוּ:
If their consecrations precede their blemishes, or passing blemishes precede their consecrations, but afterward they develop permanent blemishes (and they are redeemed), they are exempt from the [the obligations of] the firstborn and the [priestly] gifts. They do not leave [their consecrated status] to become unconsecrated to be shorn and worked. Their offspring and milk [from] after they are been redeemed are forbidden. One who slaughters them outside [the Temple courtyard] is liable. They make [their] replacement [sacred]. If they die, they are buried.
English Explanation of Mishnah Bekhorot
Are exempt from the law of the firstling, and from priestly gifts;
And they are not like unconsecrated animals to be shorn or put to work;
And [even] after they have been redeemed their young and their milk are forbidden;
And he who slaughtered them outside the sanctuary is liable;
And they render what was substituted for them [holy],
And if they died they must be buried.
An animal that is first consecrated and then becomes blemished is a consecrated animal, even though it cannot be sacrificed. Similarly, if an animal has a passing blemish, and then it is consecrated, it is a consecrated animal. These animals must be redeemed, and the money used to buy a new sacrifice. However, they remain consecrated even after redemption.
Therefore, their offspring is exempt from the laws of tithe and firstling, as are the offspring of all consecrated animals. One who slaughters them after their redemption need not give the shoulder, cheeks and stomach to the priest. Even after they are redeemed, it is forbidden to shear them or to perform any work with them. Their offspring and their milk remain prohibited, even after they have been redeemed. Indeed, it seems like the only thing that can be done with them is to eat them.
Our version of the mishnah says that if one slaughters them outside of the Temple, he is liable. This is difficult, because what it he supposed to do with them he can’t offer them as sacrifices nor can he slaughter them outside of the Temple. There is a version which reads “exempt” instead of “liable.” However, the Talmud reads “liable” and explains that this mishnah is according to Rabbi Akiva who holds that if an animal with a blemish was put onto the altar, it is not to be removed (Zevahim 9:3). Thus, although this animal should not be sacrificed, if it is put on the altar, it can be sacrificed.
If one exchanges this animal for another, the other animal also becomes holy. Consecrated animals cannot be exchanged one for the other, and if one tries to do so, the original animal remains holy and the new animal becomes holy.
Finally, if they die before they are redeemed, they must be buried, because no one can derive benefit from their meat.