How do we see from the verse of "v'Hefdah" that the Amah Ivriyah is acquired
through Kesef? Perhaps she is acquired only through Shtar, and the Gira'on
Kesef is accomplished by calculating the amount of money that was paid for
her? How can we prove from here that the payment of money itself
accomplishes a Kinyan? (RAMBAN)
(a) The TOSFOS HA'ROSH explains that the Gemara does not need a new source
to teach that an Amah Ivriyah can be acquired through Kinyan Kesef, because
the Kinyan of Kesef can be learned from the Halachah of an Eved Ivri who is
sold to a Nochri, who can be acquired through Kesef as the Gemara already
proved. Rather, the Gemara is merely looking for a source to teach that the
Kinyan of Kesef also applies to an Eved Ivri or an Amah Ivriyah who was sold
to a Jew. The verse that teaches Gira'on Kesef suffices (as a "Giluy Milsa")
to teach that the Kinyan of an Amah Ivriyah -- who is freed with Gira'on
Kesef -- is similar to the Kinyan of an Eved Ivri sold to a Nochri who is
also freed with Gira'on Kesef; just like they are both freed with Gira'on
Kesef, they are both acquired with Gira'on Kesef.
(b) The RAMBAN, RASHBA, and other Rishonim cite RABEINU TAM who explains
that the Gemara means to compare the method of acquiring an Amah Ivriyah
with the method of freeing her. If the Amah can go free with a transfer of
money (i.e. Gira'on Kesef), then she also can be acquired with a transfer of
money. (See TOSFOS RID 21b.)
(c) RABEINU TAM, as cited by TOSFOS RID (21b), suggests a different
approach. Rabeinu Tam suggests that the Gemara is assuming that the Gira'on
Kesef must be done with the Kesef of Kinyan, and not just with the value of
the Amah. According to this explanation, an Amah who is purchased with a
Kinyan Shtar is not freed with Gira'on Kesef! (Even though the Gemara on 12a
points out that an Amah cannot be sold for a Perutah because it is not
possible to perform Gira'on Kesef with less than a Perutah, nevertheless if
it is possible to perform Gira'on Kesef when she is sold with Kesef, then
she can be sold with a Shtar even though it is not possible to do Gira'on
Kesef when she is sold with a Shtar.)
This also seems to be the opinion of RASHI (DH she'Megara'as) who writes
that if the master did not acquire the Amah with Kesef, then there is
nothing with which to perform Gira'on Kesef.
The MINCHAS CHINUCH (Mitzvah 43:1) discusses the words of the Tosfos Rid at
length. He cites support for the Tosfos Rid from the Gemara (end of 11b)
which proves that the Kinyan Kesef of an Amah must be done with at least two
Perutos or a Dinar in order that it be possible to redeem her with Gira'on
Kesef. If the Gira'on Kesef is done based on the value of the Amah, and not
based on the money that was used to acquire her, then how can we prove that
the Kinyan Kesef of an Amah must be two Perutos? Perhaps a Perutah makes the
Kinyan, while the master becomes indebted to pay the rest of the money, as a
debt, to the father!
We may note, however, that the Gemara later (20b) seems to contradict the
assertion of the Tosfos Rid. The Gemara teaches that if an Eved Ivri was
sold for a sum of 200 Zuz and later his value depreciated to 100 Zuz, the
Eved may redeem himself with Gira'on Kesef by giving 100 Zuz (minus the
amount that he worked). If the Gira'on Kesef is calculated based on the
money that was used for the purchase of the Eved, rather than the true value
of the Eved, then what difference does it make if the Eved goes down in
value? The Kesef Kinyan was still 200 Zuz!
According to the Tosfos Rid, apparently this Halachah is simply a Gezeiras
ha'Kasuv which the Torah established in order to make it easier for the Eved
to redeem himself. (The Gemara there learns this Halachah from the verse,
"k'Fi Shanav" (Vayikra 25:52).)