1) A "SHALI'ACH" WHO OVERPAID
OPINIONS: The Gemara teaches that when a buyer overpays for a plot of land (for example, he pays 200 Zuz when the field is worth only 100 Zuz), the owner of the adjacent field may take it from him in accordance with the laws of "Bar Metzra," but only if he pays the buyer the full amount that the buyer himself paid (even though the buyer overpaid). The Gemara explains that even though the first buyer is considered the Shali'ach of the Bar Metzra to buy the land, and, normally, a person is not required to reimburse a Shali'ach who overpaid, nevertheless in this case he must reimburse the Shali'ach. The reason is that "Ein Ona'ah l'Karka'os" -- the law of "Ona'ah" (overcharging) does not apply to land.
What is the reason for "Ein Ona'ah l'Karka'os," and how does it explain why the Bar Metzra must reimburse the Shali'ach even though he overpaid?
(a) The RASHBAM in Bava Basra (61b, end of DH b'Bik'ah Gedolah) says that the reason why there is no law of "Ona'ah" for land is that land has unlimited value. There is no absolute market price for land, and when a person pays more than what seems to be a fair price he is not considered to be overpaying.
(b) TOSFOS in Bava Basra there (DH Shema Minah) disagrees with the Rashbam and asserts that there is no logical reason for why the law of "Ona'ah" does not apply to land. It is a Gezeiras ha'Kasuv without any apparent logic behind it. Most Rishonim agree with Tosfos.
The TOSFOS HA'ROSH here points out that the Gemara here seems to support the view of the Rashbam. The Gemara uses the rule of "Ein Ona'ah l'Karka'os" to explain why the Bar Metzra must reimburse the Shali'ach even though the Shali'ach did not benefit him with his Shelichus. The Gemara implies that the Shali'ach, in essence, did not overpay, since the purchase value of land has no limit, and the Shali'ach therefore performed no disservice to the Bar Metzra when he paid a lot of money for the land.
According to the opinion of Tosfos, however, who says that "Ein Ona'ah l'Karka'os" is a Gezeiras ha'Kasuv, how does the Gemara answer its question with the rule of "Ein Ona'ah l'Karka'os"? The Shali'ach still overpaid for the land, and thus the Bar Metzra should be able to claim that the Shali'ach did him a disservice and did not act in the capacity of a Shali'ach!
The Tosfos ha'Rosh explains that in this case of a "Bar Metzra," since the Shali'ach (the first buyer of the land) intended to buy the land for himself, when the neighbor decides to utilize his right of Bar Metzra he cannot claim that the Shali'ach did a disservice to him by paying so much for the land. At the time of the purchase, the first buyer had no intention of being the Shali'ach of the Bar Metzra. Consequently, if the Bar Metzra decides to take the field, he must take it at the existing terms and for the price that was paid.
The Tosfos ha'Rosh cites proof to this explanation of the Gemara (in contrast to the Rashbam's explanation) from the Gemara in Kesuvos (99b). The Gemara states that despite the fact that "Ein Ona'ah l'Karka'os," if a Shali'ach overpays for land that he buys for the one who appointed him, the sale is invalid. This indicates that the purchase value of land does have a limit (unlike the Rashbam's opinion) and that is why the laws of "Ona'ah" invalidate the sale. If land had an unlimited value, then even in the case of a Shali'ach who overpays for land the laws of "Ona'ah" would not apply. The Rosh thus proves that only in the case of Bar Metzra is there no claim against the Shali'ach, because at the time of the sale the first buyer intended to buy it for himself. In general, though, a Shali'ach who overpaid is at fault and the sender does not have to reimburse him. (Y. MARCUS)
2) ONE WHO SELLS HIS PROPERTY TO A NOCHRI
QUESTION: The Gemara says that when a Jew sells his property to a Nochri, the law of Bar Metzra does not apply (since a Nochri cannot be forced to fulfill the dictum of "v'Asisa ha'Yashar veha'Tov" and let the Jewish neighbor buy it). However, Beis Din places a decree of excommunication upon the seller until he accepts full responsibility for any damage that occurs to the Jewish neighbor as a result of the Nochri.
Why does Beis Din excommunicate the seller immediately when he sells his property to a Nochri, when no damage has been done yet (and might not be done)? Beis Din should wait until the Nochri causes damage to the Jewish neighbor and then should force the seller to pay for the damages, or excommunicate him if he refuses to pay. (ROSH)
(a) The ROSH answers that Beis Din penalizes the seller by immediately requiring him to write a "Shtar Shibud" -- a contract in which he commits all of his property to pay for any damage the Nochri will cause to the neighbor. Then, if the Nochri causes damage to the neighbor, the neighbor may collect the property of the seller from the time the Shtar was written.
(b) The TORAS CHAIM answers that Beis Din excommunicates him immediately as a form of punishment because he acted improperly by selling his property to a Nochri.