1) A "SHTAR MEKUSHAR" THAT IS "PASUL MID'ORAISA"
QUESTION: The Mishnah (160a) says that a Shtar Mekushar on which only two witnesses are signed is invalid, because one of the laws of a Shtar Mekushar is that it must be signed by at least three witnesses. A normal Shtar on which only one witness is signed is invalid. The Gemara here asks that it is obvious that such a Shtar is invalid; the Torah requires two witnesses in all matters of testimony! Ameimar answers that the Mishnah is teaching that just as a normal Shtar is Pasul mid'Oraisa when only one witness is signed on it, a Shtar Mekushar is also Pasul mid'Oraisa when only two witnesses are signed on it.
What does the Gemara mean when it says that a Shtar Mekushar is Pasul mid'Oraisa when only two witnesses are signed on it? A Shtar Mekushar itself is only an enactment of the Rabanan, as the Gemara says earlier (160b)!
(b) The RAMAH and RITVA answer that before the enactment of the Rabanan, 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). The Rabanan enacted that such a Shtar 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 indeed is Pasul mid'Oraisa. (See also RAMBAN to 160a.)
(b) The RASHBAM apparently maintains that a Shtar Mekushar is a valid Shtar even mid'Oraisa. The Torah gives to the Rabanan the authority to establish the definition of a Shtar, and their definition has the status of a Shtar d'Oraisa. This has two important implications. The first is that 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 Shibud Nechasim in the case of a Shtar Chov Mekushar). The second is that the witnesses who sign it serve as valid testimony and proof to what occurred.
When the Rabanan disqualified such a Shtar when it 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 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 signed 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 Meshubadim. This is what the Gemara means when it says that such a Shtar is Pasul mid'Oraisa.
However, since two witnesses are signed on the Shtar, the Shtar remains valid mid'Oraisa with regard to serving as testimony to what occurred. Thus, in the case of a Shtar Chov, the creditor may use such a Shtar as proof that the debtor borrowed money from him, and he may collect the money on the basis of this Shtar from Nechasim Benei Chorin, as the Rashbam here (end of DH u'Meshani) says (in contrast to the way Tosfos cites the Rashbam). This is also the ruling of the PISKEI HA'RID and RI'AZ.
Similarly, RABEINU GERSHOM 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 Meshubadim, Beis Din does not allow him to keep it b'Di'eved, but rather he is forced 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 that people will 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, when a child is born to a woman from a second husband after she received from her first husband such a Get that was Pasul, people should treat should treat that child like a Mamzer.)