What is the reason for "Ein Ona'ah l'Karka'os," and how does it answer our
Gemara's question concerning 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 because land has
unlimited value. There is no absolute market price for land, and thus 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 there is no law of
"Ona'ah" for land -- it is a Gezeiras ha'Kasuv without any apparent logic
behind it. Most Rishonim agree with Tosfos.
The TOSFOS HA'ROSH in our Sugya points out that our Gemara -- which says
that "Ein Ona'ah l'Karka'os" is the reason why the Bar Metzra must reimburse
the Shali'ach even though the Shali'ach did not benefit him with his
Shelichus -- seems to support the view of the Rashbam. The Gemara is saying
that the Shali'ach, in essence, did *not* overpay, since the purchase value
of land has no limit, and thus the Shali'ach did not do a 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
by citing 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 for him a disservice and therefore was not acting in the
capacity of a Shali'ach!
The Tosfos ha'Rosh explains that in our Gemara's case of a "Bar Metzra,"
since the Shali'ach (the first buyer of the land) intended to buy the land
for himself, the Bar Metzra -- when he decides to utilize his right of Bar
Metzra -- has no 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 it, 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 is buying for the one who appointed him,
the sale is invalid. We see from there that the purchase value of land
*does* have a limit (not like the Rashbam) and that is why the laws of
"Ona'ah" invalid 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. It must be, explains that the Rosh, that only in our 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)