What does the Gemara mean that a Shtar Mekushar is Pasul mid'Oraisa when it
has only two witnesses signed on it? A Shtar Mekushar itself is only an
enactment of the Rabanan, as the Gemara says earlier (160b)!
(a) The RAMAH and RITVA answer that mid'Oraisa, a Shtar Mekushar was not a
valid Shtar (because the witnesses are signed on the back of the Shtar, and
not below the text of the Shtar), and it was the Rabanan who enacted that it
may be used. Consequently, if the requirements of the Rabanan (such as
having three witnesses sign it) are not met, then the Shtar is not valid at
all, and it is indeed Pasul mid'Oraisa. (See also RAMBAN to 160a.)
(b) The RASHBAM seems to hold that a Shtar Mekushar is a valid Shtar,
mid'Oraisa. This is because the Torah gives the power to the Chachamim to
define what a Shtar is, and it then has the status of a Shtar d'Oraisa. This
has two implications. First, if a Shtar Mekushar is valid mid'Oraisa, it is
able to create some effect d'Oraisa (such as a divorce in the case of a Get
Mekushar, or a Shi'abud Nechasim in the case of a Shtar Chov Mekushar).
Second, the witnesses that are signed on it serve as valid testimony, a
proof, to what occurred.
When the Rabanan invalidated such a Shtar that has only two witnesses, that
Pesul is really only a Pesul d'Rabanan since, mid'Oraisa, the Shtar is valid
with two witnesses. The Rabanan, though, gave such a Shtar the stringency of
a Pesul d'Oraisa so that it cannot create the desired effect. Thus, if a man
gives his wife a Get Mekushar that has only two witnesses on it, the woman
is not divorced and needs another Get. Similarly, if a debtor writes a Shtar
Mekushar for his creditor, the creditor cannot use it to collect a debt from
Nechasim Meshu'abadim. This is what the Gemara means when it says that such
a Shtar is Pasul mid'Oraisa.
However, since the Shtar has two witnesses signed on it, it remains valid
mid'Oraisa with regard to serving as testimony to what occurred. Thus, in
the case of a Shtar Chov, the creditor would be able to use such a Shtar as
proof that the debtor borrowed money from him, and he would be able to
collect the money on the basis of this Shtar from Nechasim Benei Chorin, as
the Rashbam here (end of DH u'Meshani) says (and not the way Tosfos cites
the Rashbam). This is also the ruling of the PISKEI HA'RID and RI'AZ.
RABEINU GERSHOM also explains that when the Gemara says that a Shtar
Mekushar with two witnesses is Pasul mid'Oraisa, it means that the Rabanan
decreed that it be treated like a Pesul d'Oraisa, so that if the creditor
who is holding such a Shtar seizes property from the debtor's Nechasim
Meshu'abadim, we do not say that, b'Di'eved, he may keep it, but rather we
force him to return it as if the Shtar was completely Pasul.
(c) TOSFOS (DH Hacha Nami) also says that the Pesul of a Shtar Mekushar with
two witnesses is only a Pesul mid'Rabanan. The Gemara calls it a Pesul
mid'Oraisa in order for people to treat it stringently, like a Pesul
d'Oraisa, so that in the case of a normal Shtar that is Pasul (with a real
Pesul d'Oraisa), people will treat it as a real Pesul d'Oraisa and not be
lenient with it (for example, so that they will treat, as a Mamzer, any
child born to a woman from a second husband after she received from her
first husband such a Get that was Pasul).