Gemara

Rav Idi bar Avin's relative died, and left a date tree. Rav Idi and another relative (Reuven) each said 'I am the heir.'

Eventually, Reuven admitted that Rav Idi is a closer relative. Rav Chisda ruled that Rav Idi owns the tree.

Rav Idi: He must return the fruit that he ate all this time!

Rav Chisda: You have the tree only based on his admission. He says that he is the heir (and he just gave it to you for a gift)!

(Abaye and Rava): Rav Idi is right. Reuven's admission is that Rav Idi is the true heir.

33b: Yehudah claimed that he bought Levi's land, and ate the years of Chazakah. He brought witnesses that he ate the Peros for two years.

Rav Nachman: He must return the land and two years worth of Peros.

34b (Rav Nachman): If Reuven and Shimon each claimed 'I inherited this land from my fathers', Kol d'Alim Gevar (whoever overpowers will take it).

Rishonim

Rif and Rosh (3:19): The Halachah follows Abaye and Rava.

Ba'al ha'Ma'or: Likewise, if Moshe says 'I bought this land from Ploni, who bought it from you', if he cannot bring witnesses that Ploni lived there for a day, like the case of R. Chiya, he is not believed to say that he saw Ploni live there. He is believed only if he says that he saw Ploni buy it from the seller (but not through a Migo). We say 'if you saw him there for a day, surely also others did. It should have a Kol (hearsay). Bring witnesses!' The only time we use Migo is when one witness says that he ate for three years, for testimony about three years of Chazakah is difficult to bring. Therefore, he gives back the land, but he is exempt for Peros that he ate.

Rebuttal #1 (Ra'avad): If one was Machazik for three years, there is a Kol. It is harder to find testimony about one day! R. Chiya does not accept a Migo (he could have said that he himself bought it from the seller) because we do not belittle Chazakah to allow two leniencies, i.e. to claim on behalf of a buyer and Migo.

Rebuttal #2 (Milchamos Hash-m): It is harder to find testimony about one day than about three years! Since it is a short time, it is not known. Even if two or three people saw that day, Moshe cannot chain them to his leg so he can bring them to Beis Din years later! Even if Ploni was there for a night, this suffices. The Ba'al ha'Ma'or says that we do not say Migo when it should be easy to bring witnesses. We asked that one should be believed about his Bechor due to a Migo (that he could give an extra share to him before death - Bava Basra 127b). One is believed to say 'I have children' to exempt his wife from Yibum, Migo he could divorce her. It is easy to find witnesses about these!

Rosh: Reuven had seized the tree through Kol d'Alim Gevar. Some say that Rav Idi brought witnesses that he was a closer relative; some say that Reuven admitted that Rav Idi is a closer relative.

Hagahos Ashri: Reuven has a Migo. Since he could say that he did not eat Peros, he should be believed to say that he ate his own! However, since the land goes to Rav Idi, also the Peros do, for Reuven admitted. The same applies to one who ate the Peros for two years. Peros of the third year do not go back. If there were no witnesses that he ate Peros, he would not return any Peros. If we would believe that he ate for three years, we would give the land to him! If a scribe said that he wrote the names of Hash-m in a Sefer Torah Lo Lishmah, the scribe is believed to forfeit his wages, but not to disqualify the Sefer, even though these depend on each other (Gitin 54b)! All the more so we should believe him only to obligate himself, for perhaps he ate three years even though he did not buy it! There is different. Since the buyer has the Sefer Torah, it is as if witnesses say that it was written Lishmah. He is believed to forfeit his wages because admission is like 100 witnesses.

Rambam (Hilchos To'en 15:3): If Levi took possession of a field, under the assumption that he is the heir, and ate the Peros, and it was found that David is really the heir, whether witnesses said so or Levi admitted, he must return all the Peros he ate.

Lechem Mishneh: There is a bigger Chidush according to R. Chananel's text, that even though we are unsure whether or not Reuven is an heir, he must return the Peros. Surely, had the Rambam agreed to this Chidush, he would have taught it.

Ramban (33a DH v'Hahu): If the witnesses only said that Rav Idi is a relative, all should agree that Reuven is believed (about the Peros) through a Migo! We say such a Migo for one who had no witnesses of Chazakah! If the witnesses said that Rav Idi is a closer relative, why did Rav Chisda exempt Reuven? This is like Rav Chisda said elsewhere, that Migo is believed against witnesses. Had he said 'I meant that I am closer regarding the Peros, for I bought them from Rav Idi' he would be believed. Therefore, he is believed, Migo he could have said this. Abaye and Rava do not say Migo against witnesses. The Gemara did not say so, for here the witnesses did not obligate Reuven; his admission did. I say that the text says only that he claimed to be a relative; he was unsure whether or not he also inherits. The witnesses said that the deceased died childless (so Rav Idi surely inherits). We did not know if Reuven was a nephew or cousin. Rav Chisda held that (for Peros) he is believed to say that he is a nephew and he ate his share, Migo he could have denied that ate Peros. Abaye and Rava say that a Safek cannot take from a Vadai, even through a Migo.

Nimukei Yosef (DH l'Sof): Had witnesses said that Rav Idi is a closer relative, Rav Chisda would have agreed that Reuven must pay for the Peros. Had Reuven claimed that he inherits alone, Abaye and Rava would have agreed that he need not pay.

Poskim

Shulchan Aruch (CM 139:4): If two were arguing about a field, and neither was Muchzak, e.g. the owner died and David and Levi came to inherit him and each claims 'I am a relative, and I inherit', and neither has witnesses, Kol d'Alim Gevar (whoever overpowers will take it). If Levi overpowered and ate the Peros, and afterwards David brought witnesses that David is a relative, and the witnesses do not know whether or not Levi is a relative, we take it from Levi, who is a Safek relative, and give it to David, who is a Vadai relative. The same applies if David brought witnesses that David is a closer relative. Levi must return all the Peros that he ate, even if our only source that he ate is his admission.

Beis Yosef (DH v'Im): The Ri prefers R. Chananel's text, which says that later, witnesses testified that Rav Idi was a relative. Before this, Reuven took it for he was stronger. When each of two people claims that he inherited a field from his fathers, we say Kol d'Alim Gevar. Rav Chisda gave the field to Rav Idi, for he was a Vadai relative, and Reuven was a Safek. It seems that the in Rambam's text, the witnesses testified that Rav Idi was a closer relative. The Magid Mishneh says that all agree to the Rambam's law. Since he admitted that he ate, and witnesses said that Rav Idi is the heir or Reuven admitted to this, we force him to pay what he ate.

SMA (8): This is even if Levi ate for three years. Since he did not say that he bought it, three years does not prove that it is his. His only right to eat the Peros was due to (his claim of) owning the land. Since the witnesses say that the land is not his, he has no Migo, therefore, he must pay.

Prishah (7): This is unlike when Yehudah claimed that he ate Peros for three years, and witnesses testified that he ate the Peros for two years. He pays only for the two years. There, if we will believe that he ate for three years, he has a Chazakah and the entire land belongs to him. We do not apply Shuda (the judges' discretion) here, for it is possible to clarify the matter. Therefore, we say Kol d'Alim Gevar.

Shach (5): We can explain the Shulchan Aruch and Tur like the Ramban, who says that Reuven himself is unsure (if he is an heir), therefore, he is not believed through a Migo. However, in Siman 145 the Tur holds like Tosfos (that the Migo was against witnesses). The Rivash, Nimukei Yosef, Rashbam and Ba'al ha'Ma'or hold like the Ramban; this is primary.