Gemara

(Mishnah - Rebbi): If a Chatas was lost and a replacement was Hukdash, and before it was offered the Chatas was found:

If both of them are Tamim (unblemished), one is offered, and the other must die;

Chachamim say, a Chatas does not die unless it was found after Kaparah (the owner offered another in place of it).

Inference: If it was found before Kaparah, it is Ro'eh (grazes until it gets a Mum, and then it is redeemed).

Temurah 22a (Mishnah): The following must die - Vlad Chatas, Temuras Chatas and Chatas she'Mesu Ba'aleha (the owner died);

Chatas she'Avrah Shenasah (it was not offered within its first year), or if a Chatas was lost and was found with a Mum:

After Kaparah, Tamus (it must die);

Before Kaparah, it is Ro'eh. We bring a replacement with its redemption money.

The first three cases were taught by themselves, for always they must die. The last two were taught by themselves, for sometimes they are Ro'eh.

(Reish Lakish): We consider Chatas she'Avrah Shenasah as if it is in a cemetery. It is Ro'eh.

Question (Mishnah): Chatas she'Avrah Shenasah, or if it was lost and was found with a Mum, after Kaparah, Tamus.

Answer (Reish Lakish): This clause applies only to case 5. (It was lost, and found with a Mum.)

Question: This cannot be. The Seifa says that before Kaparah, it is Ro'eh until it gets a Mum. In case 5, it already has a Mum!

Answer #1 (Rabah): It was lost, and found with a Mum Over (a temporary Mum).

Before Kaparah, it is Ro'eh until it gets a Mum Kavu'a (permanent), then it is sold.

Objection (Rava): If so, it should say "Yishmor"!

Answer #2 (Rava): The Mishnah means, if (1) Chatas she'Avrah Shenasah was lost, or (2) a Chatas was lost and found with a Mum:

After Kaparah, Tamus;

Before Kaparah (in case (1)), it is Ro'eh until it gets a Mum, and then it is sold.

(Rava): If a Chatas was lost at night (and a replacement was Hukdash, and it was found in the morning), the law of Aveidah (a lost Chatas) does not apply.

This is unlike other Pesulim, for neither it nor [something bought with] its Damim may be offered at night;

A Pasul itself may not be offered, but its Pidyon may be offered.

Rishonim

Rambam (Hilchos Pesulei ha'Mukdashim 4:8): If a Chatas was lost and found before Kaparah, (even if it was found with a Mum or after it became a year old), it does not die. Rather, it is Ro'eh until it gets a Mum, and the money goes to Nedavah. If it was found after Kaparah, even if it was found with a Mum or its year passed, since it was lost at the time of Kaparah, it must die.

Kesef Mishneh: In Temurah, Rashi explains that we challenged Reish Lakish, who says that it is Ro'eh [in every case, i.e.] even [if it was found] after Kaparah. We conclude that the Mishnah teaches that if its year passed, or it was lost and found with a Mum, after Kaparah, it dies. Before Kaparah, it is Ro'eh until it gets a Mum and it is sold. Rashi explains that its year passed and it was lost and it was found Tam and the owner offered another, or it was lost and found with a Mum. Also here there are two Re'usos (problems)! When the owners offered another, it must die. Before they offered another, i.e. they do not want to offer another, it is Ro'eh, i.e. when its year passed or it was lost and found Tam. Reish Lakish said that it is Ro'eh even after Kaparah when it was never lost, so there is only one Re'usa. It seems that there are extra words in the Rambam that should be deleted. It should say "any Chatas lost and found before Kaparah does not die. Rather, it is Ro'eh until it gets a Mum, and the money goes to Nedavah.

Lechem Mishneh: Rashi says that the Mishnah is even like Rabanan who argue with Rebbi, and even if it was found before Kaparah. Rabanan say Ro'eh when a Chatas was lost and found before Kaparah. Here they agree that Tamus, for there are two Re'usos, for it has a Mum. Tosfos wrote oppositely. More Re'usos cause it to be Ro'eh! According to Rabanan, the Mishnah is only when it was found after Kaparah. It teaches a Ba'al Mum for this is a Chidush. There are two Re'usos, and we could have said Ro'eh. Rather, since it was found after Kaparah, Tamus. Before Kaparah, Rabanan always say Ro'eh. The Rambam holds like Tosfos. Below, he wrote that if it was found after Kaparah, even though it has a Mum or its year passed, since it was lost... This is like Tosfos. Even though there are many Re'usos, Tamus, since it is after Kaparah. Here, before Kaparah, he says that it is Ro'eh if it was found with a Mum. This is opposite to Rashi, who says that even Rabanan say Tamus, due to the many Re'usos. However, the Rambam's words are difficult. Here he says "even though it was found with a Mum." He holds that more Re'usos cause Re'iyah, like Tosfos, like he wrote below. Here he connotes that more Re'usos cause Tamus! There is a contradiction between these expressions "even though." We must fix the text like the Kesef Mishneh says. However, why did he explain the Rambam like Rashi? The Rambam holds like Tosfos!

Rashi (22a DH Avrah): If it was lost and found Tam, and the owner offered another, or it was found with a Mum, so there are two Re'usos, and the owner offered another, Tamus. Before Kaparah, if the owner does not want to offer another, it is Ro'eh, i.e. if its year passed or it was lost and found Tam. Reish Lakish said Ro'eh even after Kaparah in a case when it was not lost, so there is only one Re'usa.

Tosfos (22a DH Yishmor): Rashi says that he watches the Mum Over, to see if it will become a Mum Kavu'a. This is wrong. That is called Ro'eh! Rather, he waits to see if it will heal. If it does, it may be offered!

Tosfos (22a DH Iy): When its year passed, [Aveidah] does not help [to get the law of Tamus] because it is proper to be offered [for other Korbanos]. This is like the Ri, that more Re'usos bring to Re'iyah.

Acharonim

Tosfos Yom Tov (Temurah 4:1 DH she'Avrah): The Bartenura says that even Rabanan agree [that Tamus] because there are two Re'usos. If it was only lost, and found before Kaparah, it need not die. This is like Rashi and the Rambam. The Rambam did not say that the Mishnah is like Rebbi. Also in the Mishneh Torah he wrote [the Mishnah]. However, in Mishnah 3, the Bartenura and Rambam wrote that all agree that if he offered the one that was not lost, the Avudah Tamus. Here we discuss when he offered the one that was not lost! Nusach Eretz Yisrael of Perush ha'Mishnayos says that below, all agree that if he took one of the two and offered it, the other Tamus. [Chachamim and Rebbi] argue only about one who asks. We (Chachamim) tell him to offer the Avudah, and the other is Ro'eh. Rebbi holds that there is no Takanah for Kodshim. He wrote like this in Mishneh Torah. Both versions are in the Gemara. Tosfos says that more Re'usos bring to Re'iyah. Rava said that being lost at night is not considered Avudah, i.e. therefore it is not Tamus, rather, Ro'eh. Rather, the Mishnah discusses a Ba'al Mum for a Chidush. One might have thought that such a great Re'usa, Avudah and a Ba'al Mum, is like something lost at night, and it is Ro'eh. The Mishnah teaches that even so, Tamus. We can explain the Rambam like this. He explicitly wrote that if there were not two Re'usos, Rabanan would agree that it is Ro'eh. He can agree that if there is one Re'usa, Tamus. The Chidush is that even with two Re'usos, Tamus, like Tosfos explained.

Tif'eres Yisrael (Temurah 4:5): The Rambam says that if it was found before Kaparah, it is it is Ro'eh until it gets a Mum, and the money goes to Nedavah. This is difficult. If he got Kaparah with another animal after finding the Avudah, he rules that the Avudah Tamus, since he overtly rejected it [through offering another]. If he did not yet get Kaparah, the Rambam rules that he offers the Avudah! Why does he say Ro'eh? He means that the one that was not lost is Ro'eh. Alternatively, it is Ro'eh if Beis Din erred and told him to offer the one that was not lost.