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The Gemora establishes that a person is believed if he claims that fruit was sold to him. The commentators explain that this is not true in every case. A person is believed to have bought the fruit if he has already eaten them. He’s also believed to go and cut the fruit down if the owner is not present. Beis Din will not stop him even if we know the land belongs to someone else. This is the case in our Gemora. If, however, the owner is present and disputes the claim, the owner is believed. The Rashbam says if this were not the case, there would be no way to prevent people from stealing fruit.
Tosfos raises an interesting question. Our Gemora gave a case where the squatter could only give proof that he was on the land for two years. Rav Nachman therefore made him give back all the fruit which he ate during those years. Tosfos asks: Why couldn’t he keep those fruit by making use of a migo. Since he would have been believed if he said he bought the fruit, we should allow him to keep the fruit based on the claim that he bought the land!?
Tosfos answers by establishing a fundamental principle in migo. A migo is only applicable if one might make the alternative claim. In our case, however, the claimant is attempting to establish ownership on the entire property. A claim on the fruit alone would not accomplish this goal and, therefore, migo is not relevant here.