Gemara

(Rav Yehudah): If one paid the taxes and took the land (like the king's law), and the neighbor wanted it (the law of Bar Meitzra gives a neighbor the right to buy land before anyone else), we do not force him to leave.

(Chachamim of Neharde'a): We force him to leave - "v'Asisa ha'Yashar veha'Tov b'Einei Hash-m."

Question: If Reuven asked Shimon's advice about buying the land next to Shimon; and Shimon told him to buy it, must Reuven acquire the rights to buy it from Shimon (to prevent Shimon from later exercising his right to buy it)?

Answer #1 (Ravina): He need not acquire.

Answer #2 (Chachamim of Neharde'a): He must acquire.

The Halachah is, he must acquire.

Since he must acquire, if he did not acquire, (if Shimon will later say that he wanted it,) it is as if the land belonged to Shimon (from when Reuven bought it). If the value rose or dropped, Shimon pays Reuven exactly what Reuven paid for it.

If Reuven bought it for 100 and it was really worth 200: if the seller would have sold for 100 to anyone, Shimon pays 100. If the seller would have charged anyone else 200, Shimon pays 200.

Suggestion: If Reuven bought it for 200 and it was really worth 100, Shimon can say, you were like a Shali'ach to buy the land for me, but not to overpay! I need to pay you only 100.

Rejection (Mar Kashisha brei d'Rav Chisda): Chachamim of Neharde'a say that land is worth whatever one pays for it. (Shimon pays 200.)

If the neighbor needs time to raise the money (and someone else has the money ready), the seller need not wait for him;

If the neighbor asks for time to bring the money, if we estimate that he may have the money already, we wait for him. If not, we do not.

If one of the four neighbors (on the four sides) bought first, he keeps the entire land. If they come together to buy, we split the land along the diagonals, each neighbor gets the quarter bordering his property.

Rishonim

Rif and Rosh (9:23): The Halachah is, one must acquire (from a neighbor who says that he pardons the law of Bar Meitzra). Therefore, if the price went up or down, it was in the Reshus of the neighbor.

Rosh: The witnesses on the purchase write a document for the neighbor if he wants the field. It is as if the buyer bought it for him without his knowledge. No other Kinyan is needed.

Rosh (20): Rav Nachman holds that we do not dispossess one who acquired from Hefker, but he agrees to the law of Bar Meitzra. Chachamim of Neharde'a say that we dispossess him, for he should acquire somewhere else.

Rambam (Hilchos Shechenim 14:2): If the buyer (Levi) consulted with the neighbor (David), and David told him to buy it, David did not forfeit his rights. David can buy it from Levi, unless Levi acquired from him (pardon of his rights). A Kinyan is needed when David pardoned before Levi bought, but if he pardoned after Levi bought, e.g. David helped him or rented from him, or saw him build or destroy any amount and use it, and David did not protest, he pardoned, and afterwards he cannot dispossess Levi.

Rambam (3): If the neighbor was overseas, sick, or a minor, and later he returned, became healthy or matured, and later he came, we do not dispossess the buyer. If not, no one could sell land, for the buyer will say 'years later, I will lose it.' The Ge'onim ruled like this.

Rosh (23): The Rambam connotes that if the neighbor did not help him and did not (Bach - see him) use the property, he can dispossess him even after a long time. This is unreasonable. The reason for the law of Bar Meitzra is to do what is proper. We should not give the neighbor such great rights. Since he did not tell the buyer immediately after hearing about the sale that he wants the buyer to do a favor for him and let him buy it, the buyer may conclude that the neighbor did not want it or could not pay for it. It is unreasonable that he has permanent rights to dispossess him. If so, sellers will lose, for people will not want to buy. It seems that Rav Hai Gaon is correct. He wrote that once the neighbor delayed the time to go to bring money that he has ready and claim from him in Beis Din, later he cannot claim. If one says 'I will toil to obtain money', we do not wait for him. If he says 'I will bring money', we wait for him only if he is estimated to have it. This shows that Beis Din waits only this time (to bring ready money). A Gaon said that if a buyer acquired rights from one of the neighbors, the others can dispossess him. We do not say that the neighbor gave over his rights, and it is as if the buyer is one of the neighbors (when one neighbor buys, the others cannot dispossess him). Rather, a neighbor can merely pardon his own rights.

Mordechai (393): The law of Bar Meitzra applies only to fields, so he can sow or plow at once. It does not apply to houses, for everyone has an equal need for a house. Even a stone or row of date trees separates fields regarding the law of Bar Meitzra, and all the more so the wall of a house! R. Tam said so. Rivam said that in his country, they apply the law of Bar Meitzra to houses and Chatzeros. R. Simchah says that the seller has nothing to do with the neighbor. Kinyan would not help. Even Kinyan of the buyer from the neighbor does not help before the sale, for it is a mere Kinyan of words. We said in Bava Basra that a Kinyan for brothers to divide does not help, unlike each acquired a particular side. This shows that a Kinyan must be tangible. However, it suffices if the neighbor said 'go buy it for yourself. I do not want it.' The buyer is not his Shali'ach against his will.

Poskim

Shulchan Aruch (CM 175:29): If Levi consulted with the neighbor (David), and David told him to buy it, David did not forfeit his rights. David can buy it from Levi, unless Levi acquired from him.

Beis Yosef (DH Ba): If he did not acquire, David can say 'I was joking, in order to establish the proper price. Had I asked to buy it, the seller would have charged more, for he knows that it is dear to me.' The Ran and Nimukei Yosef say that he must acquire, for we consider David to have a partial Kinyan in the land itself. However, Kinyan (Chalipin) suffices, even though it is like Kinyan of words,

Shulchan Aruch (ibid): Some say that it suffices if he said in front of witnesses 'you are witnesses against me that I withdrew.' He means that he withdrew properly with a Kinyan.

SMA (52 and Prishah 46): This is like other admissions in front of witnesses. He must mean that he withdrew with a Kinyan, for if not he has an excuse (I wanted you to buy it, lest the seller change me too much).

Gra (71): This is like Odisa (an admission. It acquires, even if it is false), like R. Tam (Tosfos Bava Basra 44b DH d'Lo) and Siman 113:2.

Rema: If Levi told David to buy, and David said that he does not want to, some say that Levi acquired immediately. Levi does not become David's Shali'ach (to buy it for him) against David's will.

SMA (53): The Tur and Shulchan Aruch say that Kinyan is not needed when the seller asked the neighbor (and the neighbor told him to sell it. This implies that Kinyan is needed when the buyer asked the neighbor! The Mordechai says that waiving one's rights, and even Kinyan does not help. The Halachah does not follow him regarding this. Perhaps he would admit when the buyer offered to buy it in his own name and then give it to the neighbor, and the neighbor refused, for then he cannot say 'I wanted you to get it for the proper price.'

Gra (72): This is like one who works another's field without the owner's knowledge. He is his Shali'ach, unless the owner objected.

Shach (28): The Mordechai explains that the Gemara says 'he must acquire' to discuss after the buyer bought. Before the sale, saying 'go buy it' does not connote Bitul of the Shlichus. The Poskim are unlike the Mordechai, for they say (Sa'if 32) that after the sale a silent pardon suffices. If they held like the Mordechai, Kinyan would never help!