QUESTION: The Mishnah states that if a person writes a 100-Zuz field for his wife's Kesuvah instead of the 200 Zuz to which she is entitled, or if he does not write in the Kesuvah that all of his property is bound to the Kesuvah, he still must pay the full 200 Zuz, and she may collect from Nechasim Meshubadim (from property which buyers bought from him after the Kesuvah was written), because those elements of the Kesuvah are included in the Tenai Beis Din of a Kesuvah.
The Gemara asserts that the Tana of this Mishnah is Rebbi Yehudah, who maintains that "Achrayus Ta'us Sofer," that any Shtar that does not contain mention of Achrayus may still be used to collect from Nechasim Meshubadim, because it is assumed that every Shtar is meant to be used to collect from Meshubadim. If the Achrayus is not written in the Shtar, the scribe who wrote the Shtar simply left out mention of Achrayus by accident.
How can the Gemara assert that our Mishnah follows the opinion of Rebbi Yehudah? According to Rebbi Yehudah, every Shtar may be used to collect from Meshubadim even if no mention of Achrayus is written in it. Yet our Mishnah says that a Kesuvah may be used to collect from Meshubadim, "because it is a Tenai Beis Din," implying that other Shtaros (for which there is no Tenai Beis Din) may not collect from Meshubadim unless they include mention of Meshubadim! (RISHONIM)
(a) The RAMBAN answers that when the Mishnah says the words "because it is a Tenai Beis Din," it is only explaining the Halachah mentioned at the beginning of the Mishnah: that the woman may collect 200 Zuz even if her husband designated only a 100-Zuz field for the Kesuvah. Those words are not explaining why she may collect from Meshubadim. The Beraisa says (51b) that collection from Meshubadim is not part of the Tenai Beis Din, and that the woman may indeed forgo the right to collect from Meshubadim. Rather, the reason our Mishnah says that she may collect her Kesuvah from Meshubadim is for the same reason one may use any Shtar to collect from Meshubadim -- because "Achrayus Ta'us Sofer," like Rebbi Yehudah holds.
(b) The RITVA suggests that when the Mishnah says "because it is a Tenai Beis Din," it is not explaining why one may collect from Meshubadim. Rather it is explaining why a Kesuvah should be no different than other Shtaros. We might have thought that a Kesuvah differs from other Shtaros: In other Shtaros, we assume that the omission of Achrayus is simply an error and it is as if Achrayus is indeed written in the Shtar, but in a Kesuvah, perhaps Achrayus must be mentioned explicitly, since we find (see Gitin 49b) that the Rabanan were more lenient on the husband's estate when it comes to collecting the Kesuvah. Therefore, perhaps if there is no explicit mention of Achrayus, she may not collect from the Nechasim Meshubadim of his estate with her Kesuvah.
To counter that argument, the Mishnah says that a Kesuvah also has a quality that gives the woman more power to collect than a regular Shtar -- it is a Tenai Beis Din. This proves that we should not be more lenient with the collection of the Kesuvah, and thus we remain with the Halachah that "Achrayus Ta'us Sofer" even with regards to a Kesuvah.