Why would we have thought that a field can be bought with less than a
Perutah, when the only source for Kinyan Kesef of a *woman* is the Kinyan
Kesef of a field (as the Gemara says on 4b)?
(a) TOSFOS answers that perhaps Rebbi Asi derived that Kinyan Kesef works
for Kidushin from the verse, "v'Yatz'ah Chinam" (Shemos 21:11), and not from
the Kinyan Kesef of a field.
(b) TOSFOS suggests further that sometimes the Gemara relates the inverse of
the correct correlation (see RASHASH). That is, Rebbi Asi actually meant to
say that just like a field cannot be purchased with less than a Perutah, so,
too, the Kidushin of a woman cannot be done with less than a Perutah. This
is the logical correlation, since the Kinyan Kesef of a woman is derived
from the Kinyan Kesef of a field.
(c) The TOSFOS HA'ROSH answers that according to the Girsa of Rashi
(beginning of 3b), Kidushin depends not only on what we learn from the
purchase of a field, but also on the self-respect of a woman. A woman cannot
become Mekudeshes with less than a Perutah, because she considers it
disrespectful and she does not fully consent to the Kidushin (see Insights
there). Hence, even though Kidushin cannot be done with less than a Perutah,
we might have thought that a field can be bought with less than a Perutah.
Therefore, Rebbi Asi must teach that a field cannot be bought for less than
a Perutah.
Tosfos does not accept this answer, probably because he is following his
opinion expressed earlier (3a) where he rejects Rashi's Girsa and says that
the amount of the Kesef Kidushin does not depend on the woman's
self-respect.
In a similar vein, HE'OROS B'MASECHES KIDUSHIN suggests that the RAMBAM in a
number of places seems to differentiate between the Kesef used to purchase a
field and the Kesef used to marry a woman. For example, a man can be
Mekadesh a woman by lending her money (as long as he says so at the time he
gives her the loan; see Insights to 5b), while a field cannot be purchased
in such a way. Also, a woman cannot be acquired through a "Matanah Al Menas
l'Hachzir," a gift given on condition that it be returned to the giver, but
a field could be acquired in such a way.
He explains that the Kidushin of a woman is done through the *Hana'ah* and
not through the Kesef itself, since a woman is not an item of set value like
a field. Rather, the Hana'ah that she receives causes her to allow herself
to become Mekudeshes to the man (see AVNEI MILU'IM 29:2). Hence, we might
have thought that a woman is not Mekudeshes with less than a Perutah because
that is not considered Hana'ah, while a field could be purchased with less
than a Perutah because that is still money, even though it does not
constitute Hana'ah. Rebbi Asi teaches that less than a Perutah is also not
considered money and cannot be used to buy a field.
(d) The RASHASH says that Rebbi Asi means merely that even though the
*Mishnah* did not discuss how much money is needed for the purchase of a
field, like it discussed how much money is needed for the Kidushin of a
woman, nevertheless the minimum purchase price of a field is still a
Perutah.
Perhaps the Rishonim did not find this answer acceptable is because Rav Asi
should then have made clear the connection between these two Halachos. He
should have said, "*The reason* a woman must be Mekudeshes with a Perutah
(as the Mishnah says) is *because* a field must be purchased with at least a
Perutah (and Kidushin performed with Kesef is learned from the way a field
is purchased)."