Gemara

37b (Mishnah - Admon): If Levi went overseas, and the path to his field was lost, he receives a short path to his field;

Chachamim say, he must pay whatever his neighbor charges for a path, or else he must fly (i.e. he may not walk) to his field.

Question: We understand Admon (surely, Levi used to have a path, and the neighbor took it). Why do Chachamim argue?

Answer (Rav Yehudah): The case is, he was surrounded by four neighbors on the four sides of his field.

Question: If so, what is Admon's reason? (He has no Vadai claim against any neighbor!)

Answer (Rava): If every neighbor owned his land (when Levi left), or if all bought from one person who used to own all the property surrounding the field, all agree that they can push off Levi. They argue when Levi was surrounded by four neighbors, and one person bought all their land.

Admon says, in any case the path to his field is by the buyer!

Chachamim say, the buyer says 'accept my terms. If you protest, I will return the deeds to the original owners (undo the sales), and you will be unable to force them to give you a path.'

Rishonim

The Rif and Rosh (Kesuvos 64b and 13:13) bring our Gemara verbatim.

Rambam (Hilchos To'en 15:11): If Levi went overseas and lost the path to his field, whether the field was initially surrounded by different neighbors in all four directions, or his four current neighbors bought from one seller, each neighbor can say that perhaps the path is by someone else. The same applies if one person (David) bought the surrounding four fields from four sellers. David can say 'if I would return the document to each seller (to undo the sale), you would have no path through any of them. I bought from each all of his rights.' However, if David owned all the surrounding fields from when Levi left until now, Levi says 'in any case my path is by you.' Levi is entitled to a short path; David selects it.

Magid Mishneh: The Halachah follows Chachamim.

Rambam (ibid.): If Levi was Machazik in a path and says that it was his, a clear proof is required to remove him.

Magid Mishneh: The Rambam discusses when Levi's Chazakah is the proper Shi'ur (three years).

Rebuttal (Kesef Mishneh): If so, this is obvious! Rather, after losing it he said that he recognized it. Even if he was Machazik against David's will, since we know that he had a path, David cannot remove him without a proof.

Defense (Drishah CM 148:1): This is not obvious! Since Levi knows that he had a path but David can dispel him, Levi feels justified to take any path he sees fit. One might have thought that his Chazakah is without a claim.

Rebuttal (of Kesef Mishneh - Lechem Mishneh): Granted, if David was quiet, this is like an admission, even if it was not three years. But a Chazakah by force against David's will is not a Chazakah! The Kesef Mishneh explains that this law (if Levi was Machazik...) is only when we know that he had a path by David. According to the Magid Mishneh, it is even in the Reisha when four neighbors surround him.

Milchamos Hash-m (Bava Kama 3a, b'Sof): If Reuven bought properties of different qualities from Shimon, and Shimon's creditor Yehudah was authorized to collect from the property, Reuven can threaten Yehudah 'if you demand the better property, I will return the document for the lower quality property to Shimon, and you will get it.' If Reuven does not say so himself, we do not say so for him. When Levi lost his path, David need not say this. If David would return the documents Levi would not get anything. David bought all the rights of the buyers, so Levi has no claim.

R. Akiva Eiger (Teshuvah 2:62): The Rambam says that David bought from each seller all of his rights and he mentions the threat to return the documents!

Mishneh l'Melech: A case occurred (in Chutz la'Aretz) which Levi bought a field from a Nochri who bought it from Moshe, and Moshe was contesting the sale. If Moshe could not retrieve it from the Nochri according to the law of the kingdom, Levi can say 'I bought from him all rights he had to it', because the Halachah follows Chachamim.

Poskim

Shulchan Aruch (CM 148:1): If Levi went overseas and lost the path to his field, whether he was surrounded by four neighbors on the four sides of his field, or his four current neighbors bought from one owner, each of them can say that perhaps the path is by someone else. Therefore, he must pay whatever the neighbors charge for a path, or else he must fly to his field.

Prishah (DH u'Mah she'Chosav): Even when they bought from one, the seller could have given a path wherever he wanted. For any field, he could say 'I will not give to you from here'.

Shulchan Aruch (2): Similarly, if David bought the surrounding fields from four sellers, Levi has no path. David can say 'if I would return the document to each seller, you would have no path through any of them. I bought from each all of his rights.'

Beis Yosef (DH Mi): The Rosh (Bava Kama 1:3) says that whenever the Gemara says that Ploni can threaten to return the document, even if he does not return it, it is as if he returned it. The Rambam explain that this is because he bought from each seller all of his rights.

Prishah (DH Achazir): We consider it as if Ploni returned the document only when he would gain by doing so. Here he would gain, for he would receive the full price of all the fields (without deducting for a path), since each seller would be able to say that the path was not by him.

Shach (2): If the four sellers died, David must give to Levi a path.

Rebuttal (Teshuvas Rema 109 DH Ach): This applies only to one who bought a borrower's property, for his claim is that he left property for the buyer to collect from, and orphans need not pay the debt. It does not apply here, for the buyer bought all the sellers' rights.

Shulchan Aruch (ibid.): However, if David owned all of the surrounding fields from when Levi left until now, Levi says 'in any case my path is by you'. Levi is entitled to a short path; David selects it.

SMA (3): The same applies whenever there has always been one owner, even if it is not the original owner.

Shulchan Aruch (ibid.): If Levi was Machazik in a path (Rema - like the law of Chazakah) and says that it was his, a clear proof is required to remove him.

Shach (3): This refers to the entire Siman, even when his neighbor(s) need not give to him a path. Nevertheless, three years are not required.