Gemara

R. Elazar citing R. Hoshaya: If an Ashir transgressed Tum'as Mikdash and brought Korban Oni, he was not Yotzei.

R. Chana citing R. Hoshaya: He was Yotzei.

Question (Mishnah): If a Metzora Oni brought Korban Ashir, he was Yotzei; if an Ashir brought Korban Oni, he was not Yotzei.

Answer: Metzora is different, for it says "Zos" (he must bring precisely this).

Question: If so, also a Metzora Oni who brought Korban Ashir should not be Yotzei!

Answer: "Toras" allows this:

Beraisa: "Toras" - if a Metzora Oni brought Korban Ashir, he was Yotzei.

Suggestion: Perhaps if an Ashir brought Korban Oni he was Yotzei!

Rejection: "Zos".

Question: We should learn from Metzora (to teach that no Ashir is Yotzei with Korban Oni)!

Answer: "V'im Dal Hu" limits the law (an Ashir is not Yotzei with Korban Oni only regarding Metzora).

Nidah 70a: If the Korbanos of two Metzora'im became mixed with each other, and one of them was offered, and one Metzora died, what does the other one do?

Answer (R. Yehoshua): He gives his property to someone else, and becomes an Oni. He brings Chatas ha'Of mi'Safek.

Question: Still, he may not offer the Asham (and it is Me'akev his Taharah)!

Answer (Shmuel): The case is, he already offered his Asham (the Ashamos of both were offered, or the Ashamos were never mixed).

Objection (Rav Sheshes): This is not like any Tana!

It is not like R. Yehudah, who says that the Asham is Kove'a (his wealth at the time he brings the Asham determines whether he brings Korban Ashir or Oni. If he was wealthy then, giving away his property afterwards does not help.)

It is not like R. Shimon, who says that the Chatas is Kove'a. R. Shimon permits one to stipulate and bring an Asham mi'Safek (if I am exempt from Asham, this lamb is a Shelamim. According to R. Shimon, we need not say that the Asham was already offered.)

Chachamim forbid to bring an Asham on condition, for it could cause Kodshim to be wasted. Perhaps it is really Shelamim and we should be allowed to eat it for two days, but we may not eat it on the second day because (to us) it is a Safek Asham,

Answer: Shmuel holds like R. Shimon regarding Korban Metzora (Chatas is Kove'a), but he holds like Chachamim regarding an Asham on condition.

Rishonim

Rambam (Hilchos Shegagos 10:13): If an Oni brought Korban Ashir he was Yotzei; if an Ashir brought Korban Oni he was not Yotzei.

Rambam (Hilchos Mechusarei Kaparah 5:10): If the Korbanos of two Metzora'im became mixed up, and one Chatas was offered and one of the Metzora'im died:

The live Metzora cannot bring Chatas Behemah, for perhaps his was brought, and Chatas Behemah may not be brought mi'Safek;

He cannot bring Korban Oni, for an Ashir who brought Korban Oni was not Yotzei.

Therefore, he gives all his property to someone else to become an Oni and brings Chatas ha'Of mi'Safek; the bird is not eaten. This enables him to eat Kodshim.

Question (Kesef Mishneh and Ri Korkus): The Rambam rules like R. Yehudah, that the Asham is Kove'a. If so, it does not help to give away his property afterwards! Surely the Asham was already offered, for if the Chatas was offered before the Asham it would be invalid, and the Metzora would simply bring all his Korbanos.

Answer #1 (Lechem Mishneh): The Rambam rules (Mechusarei Kaparah 1:5) that the Asham is not Me'akev Kaparah, unlike Shmuel.

Rebuttal (Kesef Mishneh and Ri Korkus 1:5): This is a textual mistake in the Rambam. Surely, he agrees that the Asham is Me'akev.

Answer #2 (Ri Korkus): Perhaps R. Yehudah agrees that when there is no other solution, Hakravas ha'Chatas is Kove'a (the Olah).

Question: The Gemara (Nidah 70b) said that Shmuel holds like R. Yehudah who forbids to bring Asham on condition and like R. Shimon who says that Hakravas Chatas is Kove'a; it did not give this answer!

Answer (Ri Korkus): Perhaps the Gemara merely shows that we can explain Shmuel even if R. Yehudah would not agree in this case. Alternatively, the Gemara means that Shmuel holds like R. Shimon (that Chatas is Kove'a) when there is no other solution. If not, the Halachah normally follows R. Yehudah against R. Shimon.

Chidushei Mahari'ach (Sof Nega'im): When there is no other solution Shmuel relies on R. Shimon against an individual (R. Yehudah, who says that Asham is Kove'a). He does not rely on R. Shimon against Chachamim (who forbid Asham on condition).

Answer #3 (Tosfos Chodoshim Sof Nega'im): The Rambam holds that for the sake of his Tikun we may permit either to bring Asham on condition (even though this diminishes the time to eat it) or to bring Korban Oni. Since the Halachah will not apply until Moshi'ach comes, the Rambam did not decide which is allowed.

Objection (Tosfos R. Akiva Eiger Sof Nega'im): If the Asham was already offered, he is not Yotzei with Korban Oni. If it was not offered, Hakravas ha'Chatas is Kove'a (the Olah).

Sefer ha'Chiruch (123): If an Oni brought Korban Ashir he was not Yotzei. Since Hash-m had mercy on him to bring Korban Oni, it is improper for him to compel himself to bring Korban Ashir.

Rebuttal (Hagahos Mishneh l'Melech and Minchas Chinuch): The Mishnah says regarding Metzora that he was Yotzei. The same applies to other Mechusarei Kaparah. R. Shimshon says that it is praiseworthy for him to bring Korban Ashir, and he will be blessed for doing so!

Defense #1 (Sefas Emes Yoma 41b DH Gufa): Regarding Metzora an Oni and Ashir both bring an Olah and a Chatas (albeit an Oni brings birds and an Ashir brings animals), therefore an Oni is Yotzei with Korban Ashir. We cannot learn to cases when an Ashir brings only Chatas Behemah, for the Oni was obligated to bring Chatas ha'Of and Olas ha'Of.

Defense #2 (Aruch l'Ner Kerisus 28a DH veha'Tanya) Question: Seemingly, the Beraisa should first expound that an Ashir is not Yotzei with Korban Oni. Then, we might have thought to learn from this that likewise an Oni is not Yotzei with Korban Ashir, and "Toras" would teach that this is not so. Why does the Beraisa first expound that an Oni is Yotzei with Korban Ashir? This was obvious!

Answer: Indeed, this teaches that the 'Sevara Rishonah' (before Drashos) is that an Oni is not Yotzei with Korban Ashir (and all the more so vice-versa). Regarding Metzora the Torah teaches that an Oni is Yotzei with Korban Ashir. It did not teach this regarding other cases of Oleh v'Yored, we apply the Sevara Rishonah that an Oni is not Yotzei with Korban Ashir.

Acharonim

Kovetz Shi'urim (Kesuvos 323): The Metzora is allowed to give away his property and bring Korban Oni to Metaher himself. One is not obligated to give up more than a fifth of his wealth for a Mitzvas Aseh. Some say that he gives away his property on condition that the recipient will return it. It is not clear if he is considered an Oni in this case (328).

Question: If Leah brought Chatas ha'Of, some say that her heirs must bring the Olas ha'Of even if she did not separate it, for her property was Meshubad. If so, how does it help for a Metzora to give away his property? There is a Shibud on it to bring Korban Ashir!

Answer #1 (Kovetz Shi'urim, ibid.): Here the Shibud is ineffective because Hekdesh cannot take from the one who received the property, for perhaps the Metzora was already Yotzei the Korban.

Question: This is unlike Tosfos (Zevachim 12 Sof DH u'Shma), who says that (in any case) an Ashir can make his property Hefker and bring Korban Oni!

Answer #2: We collect from Meshubadim regarding regular debts, for even if the debtor had no money he would be obligated; his property is like an Arev for him. However, someone without money is exempt from Korban Ashir, therefore we do take from his former property to bring Korban Ashir.