(a) TOSFOS and the TOSFOS HA'ROSH explain that even if an Eved Ivri cannot
marry a Shifchah, we might think that the Gezeirah Shavah of "Sachir Sachir"
teaches that he can become Nirtza even without being able to marry a
Shifchah, since we must compare him to an Eved sold by Beis Din. The Tosfos
ha'Rosh brings proof to this from the Gemara (17b) that learns from a verse
that an Amah Ivriyah cannot become Nirtza. Why do we need a verse if an Amah
Ivriyah cannot marry a Shifchah Kena'anis (or Eved Kena'ani)? It must be
that we would learn from the Hekesh which compares Eved Ivri to Amah Ivriyah
that all of the Halachos are the same, and that the Amah Ivriyah can become
Nirtza even though she cannot marry an Eved Kena'ani.
However, the RASHBA (15a) refutes this proof. He explains that we need a
verse to exclude Amah Ivriyah from the Halachah of Nirtza, because otherwise
we might have learned from the verse, "Af la'Amascha Ta'aseh Ken" (Devarim
15:16), that an Amah Ivriyah can become Nirtza (as the Gemara says on 17b).
(b) The SHA'AR HA'MELECH (Hilchos Avadim 3:3) suggests that even though an
Eved who sells himself normally cannot marry a Shifchah Kena'anis, if the
person who sold himself was a Mamzer then he is permitted to marry a
Shifchah, since a Mamzer is always permitted to marry a Shifchah Kena'anis.
Hence, Perhaps the verse is needed to teach that even a Mamzer who sold
himself cannot become Nirtza, even when he marries a Shifchah Kena'anis.
The Acharonim reject this answer, because it is clear from the verse that
the Eved's marriage to the Shifchah is part of the reason why he does not
want to go free (since, upon going free, he will become prohibited to her).
If the Eved is a Mamzer, then he is permitted to the Shifchah even when he
is not an Eved, and thus going free will not prohibit her and it is not a
reason why he would not want to go free.
(c) The RITVA (14b) answers that even though the master cannot force the
Eved who sold himself to marry a Shifchah, nevertheless the Eved is
permitted to marry one if he wants.
The Ritva proves that an Eved who sold himself is permitted to marry a
Shifchah from the fact that he needs a Get Shichrur in order to go free. He
needs a Get Shichrur because "Eved Ivri -- Gufo Kanuy," the body of the Eved
is owned by the master (as the Gemara says on 16a). If an Eved Ivri who sold
himself is not permitted to a Shifchah, in what way is his Guf owned by the
master?
(This assertion of the Ritva is difficult to reconcile with the verses. The
same verse that teaches that an Eved sold by Beis Din is permitted to marry
a Shifchah teaches that such an Eved is given a Shifchah against his will.
Why should we exclude an Eved who sold himself from only half of what the
verse teaches, and not from everything? Apparently, the fact that the Torah
requires a Get Shichrur in order to free an Eved who sold himself is the
source that such an Eved is permitted to a Shifchah, according to the
Ritva.)
The Ritva is alluding to the RAMBAN's interpretation of the Gemara (16a)
that says that "Eved Ivri -- Gufo Kanuy." The Ramban there asks why does the
Gemara say that the master must give the Eved Ivri a Get Shichrur and not
just tell him to go, because of the fact that the Eved Ivri's Guf belongs to
the master? If it is a monetary Kinyan just like anything else that the
master owns, then he should be able to send away the Eved without a Get
Shichrur, just like he can make any of his property Hefker or give it away!
The Ramban (cited by the RASHBA) answers that a Kinyan ha'Guf of an Eved is
not a monetary Kinyan, but a "Kinyan Isur," similar to the Kidushin of a
woman, and similar to the Kinyan of an Eved Kena'ani (see Insights to Gitin
XX). Just like the Kinyan of Kidushin of a woman and the Kinyan of an Eved
Kena'ani has Halachic ramifications of Isur (to prohibit her to the world,
or to prohibit the Eved Kena'ani to a Bas Yisrael), so, too, the Kinyan of
an Eved Ivri has Halachic ramifications -- it permits him to a Shifchah
Kena'anis. Such a Kinyan cannot be annulled by simply being Mafkir the Eved
(which is an act of a monetary nature), but rather it needs a Halachic
removal (such as by giving a Shtar Shichrur).
The Ritva interprets the Kinyan ha'Guf of an Eved Ivri like the Ramban. He
therefore wonders why an Eved who sold himself should need a Get Shichrur if
there are no Halachic ramifications to his Kinyan! This is what leads the
Ritva to conclude that the Kinyan of an Eved who sold himself does have
Halachic ramifications, in that he, too, becomes permitted to a Shifchah.
However, the RASHBA (16a) reaches the opposite conclusion. He writes that it
would seem, according to the Ramban, that an Eved who sold himself may
indeed be sent away without a Get Shichrur. (Perhaps the Ritva maintains
that we would learn from the Gezeirah Shavah of "Sachir Sachir" that a an
Eved Mocher Atzmo also goes free with a Shtar and that his master has a
Kinyan ha'Guf on him.)
RASHI and TOSFOS (16a) seem to have understood that the Kinyan ha'Guf of an
Eved Ivri is not necessarily a Kinyan of Isur, like the Ramban understood,
but rather it is a monetary Kinyan. Indeed, the master could be Mafkir the
Eved Ivri and make him free without giving him a Get Shichrur. When the
Gemara says that the master cannot send the Eved away unless he gives him a
Shtar Shichrur, it means that he cannot forgive (be "Mochel") the obligation
of the Eved Ivri to serve him, since that obligation is not just a Shi'abud
(a commitment for the future that is forgivable), but rather the master
actually owns the Eved with regard to his services, and therefore he must
make a formal Kinyan to the Eved when he wants to give him back the rights
to his services (see Rashi there, 16a, DH Gufo Kanuy).