Gemara

51a (Mishnah): If a man did not write a Kesuvah to his wife, it is an enactment of Beis Din that a virgin collects 200 and a non-virgin collects 100.

This is like R. Meir.

54b (Mishnah - R. Yehudah): One may write a Kesuvah of 200 to a virgin, and she writes (a receipt) that she received 100 (even if she did not). He may write 100 to a non-virgin, and she writes that she received 50;

R. Meir says, if anyone reduced the Kesuvah from 200 for a virgin or from 100 for a non-virgin, their Bi'ah is like Zenus.

56b - Inference: Anyone connotes that even if he stipulated, the stipulation is void and the Kesuvah really is 200, but since she thinks that it is not 200, the Bi'ah is Zenus.

Question: R. Meir holds that a stipulation contrary to Torah is void. This implies that a stipulation contrary to a mid'Rabanan law is valid!

Answer: R. Meir holds that the Kesuvah is mid'Oraisa.

(Beraisa - R. Meir): If anyone reduces the Kesuvah of a virgin from 200, or of a non-virgin from 100, the Bi'ah is Zenus.

R. Yosi says, he is permitted;

Contradiction (Beraisa): To fix the world, we do not designate Metaltelim (to be the sole source of payment) for a Kesuvah;

Objection (R. Yosi): How does this fix the world? Their value is not fixed, and they depreciate!

The first Tana allows designating Metaltelim only if he accepted Acharayos (responsibility to pay from other property if they will be lost). R. Yosi forbids even in this case. Their value is not fixed, and they depreciate!

R. Yosi is concerned lest they depreciate. All the more so he forbids stipulating to definitely decrease the Kesuvah!

Answer: She does not know that Metaltelim will depreciate, and does not intend to pardon the loss. Here, she knowingly pardoned the decrease!

Rami bar Chama's sister was married to Rav Avya. She lost her Kesuvah.

Rav Yosef (citing Shmuel): R. Meir requires her to have a Kesuvah of 200, but Chachamim permit one to keep his wife for two or three years without a Kesuvah!

Abaye (citing Shmuel): The Halachah follows R. Meir in his decrees!

Rav Yosef: If so, he must write for her another Kesuvah.

Rishonim

The Rif and Rosh (5:7) bring the Gemara verbatim (except for the contradiction in R. Yosi).

Ran (DH Masnisin): If the stipulation were valid and her Kesuvah is only 100, obviously R. Meir calls this Bi'as Zenus! This shows that R. Meir holds that the stipulation is void. Even though her Kesuvah is 200, it is Bi'as Zenus because she is not confident of collecting.

Hagahos Ashri: Rashi says that 'R. Meir's decrees' refers to matters in which he is more stringent than Divrei Torah. This is wrong. It is only when he forbids one matter due to another (Eruvin 47a). BaHaG and the She'altos say that the Halachah follows his decrees, but not his fines (Yevamos 37a).

Rambam (Hilchos Ishus 10:10): If one married a woman without writing a Kesuvah for her, or if he wrote it and she lost it, or she pardoned or sold it to him, he must write Ikar Kesuvah to her again if he wants to keep her. A man may not have a wife, even for a moment, without a Kesuvah.

Rambam (12:8): If a man stipulated to reduce the Ikar Kesuvah, or he wrote Ikar Kesuvah of 200 or 100 and she falsely wrote that she received part, the stipulation is Batel. This is because if anyone reduced the Kesuvah from 200 for a virgin or from 100 for a non-virgin, their Bi'ah is like Zenus.

Ran (DH Gemara): A woman collects due to Tanai Beis Din even without a document, but she is not confident of collecting. She worries lest he will claim that he paid her. Therefore, it is Bi'as Zenus. We hold like R. Meir regarding a Kesuvah that is too small, but not regarding a stipulation. R. Meir invalidates the stipulation because he holds that Kesuvah is mid'Oraisa. We hold that it is mid'Rabanan. Even regarding mid'Oraisa laws, we hold that a monetary stipulation is valid. R. Yehudah holds that Chachamim strengthened their enactments more than Torah law, and a stipulation about mid'Rabanan laws is invalid, R. Yosi argues. The Halachah follows R. Yosi against R. Yehudah. However, the Rambam says that the stipulation is Batel even if she falsely writes that she received part.

Magid Mishneh: The Rambam applies the rule (the Halachah follows R. Meir's decrees) to a stipulation, but not for R. Meir's reason. Rather, Kesuvah is mid'Rabanan, and Chachamim strengthened their enactments like Torah law.

Question (Beis Shmuel EH 66:9): A stipulation against Torah law takes effect!

Answer (Lechem Mishneh): The text of the Magid Mishneh should say 'they strengthened their enactments more than Torah law.'

Magid Mishneh: Nevertheless, it is Bi'as Zenus because she is not confident of collecting. If he does not intend to keep her, his stipulation is valid. We find that a woman can pardon her Kesuvah to her husband. Some say that the Halachah follows R. Meir only regarding Bi'as Zenus, but not regarding stipulations.

Beis Shmuel (ibid.): The Gemara (Bava Kama 89a) said that if a married woman damaged, she does not sell the Tovas Hana'ah of her Kesuvah to pay the damage, for surely, she will pardon the Kesuvah to please her husband. The Rambam says that she cannot pardon it! It seems that the Rambam says only that a stipulation at the time of Kidushin or Nisu'in is Batel, for it is contrary to Torah.

Poskim

Shulchan Aruch (EH 66:3): If one wrote a Kesuvah for his wife and she lost it or (Rema - specifically) wrote that she received, he must write Ikar Kesuvah to her again. A man may not have a wife, even for a moment, without a Kesuvah.

Chelkas Mechokek (13): The Rema added the word 'specifically'. This is like the Mordechai, who say that pardon of a Kesuvah during the marriage is invalid if it was not written, for she can say that she was joking.

Question: Even if her pardon was void, she is not confident of collection! This is like a stipulation to reduce her Kesuvah, which R. Meir says is void, nevertheless it is Bi'as Zenus!

Answer (Beis Shmuel 9): She does not think that her pardon helps. The Mechaber holds that even written pardon does not take effect. However, a new Kesuvah is needed to assure her of collection.

Question: A Mishnah (51a) says that even without a written Kesuvah, a woman collects 200 or 100 by Rabbinic enactment. We established it to be R. Meir. R. Meir forbids this, for she is not confident of collection!

Answers (Beis Yosef DH v'Im): The Mishnah is in a place where they do not write Kesuvos. All women rely on the Tanai Beis Din. Alternatively, it is in a place where they write Kesuvos. The Mishnah teaches that she collects. Indeed, it is Bi'as Zenus!

Rema: When she lost her Kesuvah, he must write a Kesuvah as big as she had before.

Prishah (3): He writes only Ikar Kesuvah, 200 or 100.

Beis Shmuel (10): Seemingly, he should need to write only 100, fornow she is not a Besulah.

Rema: If a woman cannot be divorced b'Al Korchah (against her will, e.g. a rapist) one need not write a Kesuvah. If so, in our lands where we do not divorce b'Al Korchah due to R. Gershom's Cherem, one could be lenient about writing a Kesuvah. However, this is not the custom, and one should not deviate.

Beis Shmuel (11): It is different when there is an Isur mid'Oraisa to divorce her.