Gemara

(Rav Nachman): Chachamim enacted that a virgin gets a Kesuvah of 200 Zuz, and a widow gets a Kesuvah of 100. They also enacted that if he claims Pesach Pasu'ach (he felt that she had no Besulim), he is believed.

(Beraisa #1): Since the Kesuvah is an enactment of Chachamim, she collects only from Ziburis (lowest quality land).

R. Shimon ben Gamliel says, the Kesuvah is mid'Oraisa.

Contradiction (Beraisa #2): "Money... like the Mohar of virgins" teaches that the Mohar (Kesuvah) of a Besulah should be like the fine for a seducer (50 Shekalim). This was Chachamim's source in the Torah for the Kesuvah;

R. Shimon ben Gamliel says, the Kesuvah is only mid'Rabanan.

Answer #1: We must switch the opinions in Beraisa #2. We don't switch Beraisa #1, for elsewhere, R. Shimon ben Gamliel says that the Kesuvah is mid'Oraisa.

(Mishnah - R. Shimon ben Gamliel): One pays the Kesuvah with coins of Keputkiya (which are larger).

Answer #2: R. Shimon ben Gamliel taught all of Beraisa #2. It means, 'this is the support that Kesuvah is mid'Oraisa. The Kesuvah of a widow is only mid'Rabanan, like R. Shimon ben Gamliel says.'

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 unlike R. Yehudah, who permits her to write a receipt that she received half (of this amount, even if she received nothing).

56a - Question: R. Yehudah implies that she must write that she received 100. It is not enough for her to say so. He holds that monetary stipulations (even against the Torah) are valid!

Answer: He holds that Kesuvah is mid'Rabanan, and Chachamim strengthened their words more than Torah laws.

110b (Mishnah): A man can pay the Kesuvah in coins of Eretz Yisrael (which are smaller than those of Kaputkiya) if he married or divorced her in Eretz Yisrael;

R. Shimon ben Gamliel says, if he married her in Kaputkiya and divorced her in Eretz Yisrael, he pays coins of Kaputkiya.

(Rabah): Chachamim hold that Kesuvah is mid'Rabanan, therefore, they are lenient to allow paying smaller coins. R. Shimon ben Gamliel holds that Kesuvah is mid'Oraisa. (Therefore, he pays with coins from the place he married her.)

(Beraisa): A loan document is paid with coins of the place where it was written. If no place is written, it is paid with coins of the place of collection. If it says only 'Kesef', the borrower pays whatever he wants. This does not apply to a Kesuvah.

(Rav Mesharshiya): 'Thisdoes not apply to a Kesuvah' refers to the Reisha (one pays with coins of the place it was written). This is unlike R. Shimon ben Gamliel.

Yevamos 89a: Kesuvah was enacted lest it be light in a man's eyes to divorce his wife. If a woman remarried based on a false report of her husband's death, she has no Kesuvah, for we want her to be divorced!

Sanhedrin 21a - Question: "Va'Yikach David Od Pilagshim v'Nashim" - what is the difference between Pilagshim and Nashim (wives)?

Answer (Rav Yehudah): Wives have a Kesuvah and Kidushin, Pilagshim do not have a Kesuvah nor (some texts - but they have) Kidushin.

Rishonim

Rif and Rosh (65b and 13:20): Rav Mesharshiya taught that the Reisha of the Beraisa does not apply to a Kesuvah. The Tana holds that Kesuvah is mid'Rabanan. This is the Halachah. (Rosh - R. Tam disagrees; see below.)

Rosh (1:19): Rav Nachman said that Chachamim enacted that a virgin gets 200. This connotes that Kesuvah is mid'Rabanan. Yevamos 89a says similarly. Kesuvos 51a says that if one did not write a Kesuvah, it is an enactment of Beis Din that a virgin collects 200 and a non-virgin collects 100. Another proof is that she collects only from low quality land. (Creditors collect from medium quality land.) Why is the custom in Ashkenaz to write in the Kesuvah '200 Zuz that befit you mid'Oraisa'? R. Tam says that we rely on R. Shimon ben Gamliel. The Halachah always follows him in the Mishnah (with three exceptions. This is not one of them.) However, the Halachah (normally) follows Rav Nachman in monetary laws! Here the Halachah does not follow Rav Nachman, for Rav Ashi, who is Basra, disagrees about Pesach Pasu'ach (12a).

Hagahos Ashri: R. Tam learns from Rav Nachman that if Kesuvah were mid'Oraisa, one would not be believed about Pesach Pasu'ach.

Mishneh l'Melech (Hilchos Ishus 11:8 DH umi'Kol Makom Ika): The Torah would not enact 200 for a Besulah in vain, i.e. if a man were believed about Pesach Pasu'ach. Rav Nachman holds that Chachamim were not concerned for this, and enacted 200 anyway.

Ran (DH ul'Inyan): The Rif disagrees with the rule that the Halachahalways follows R. Shimon ben Gamliel in the Mishnah.Amora'im argue about it.

Rosh (ibid.): The Halachah follows R. Akiva, who obligates a fine for raping a girl divorced from Eirusin (Kesuvos 40a). R. Yosi ha'Galili argues, and uses "Kesef Yishkol k'Mohar ha'Besulos" to equate the fines of a rapist and seducer (both are 50 Shekalim). R. Akiva must expound "Mohar ha'Besulos" to teach that mid'Oraisa, a Kesuvah is 50 Shekalim (10a)! However, Rashi says that they do not argue about the meaning of 'Mohar', so this is not a proof.

Rosh (ibid.): Kesuvah is an enactment of Beis Din where it is not mid'Oraisa, e.g. for a widow. R. Chananel rules like Rav Nachman. Here the Halachah is unlike R. Shimon ben Gamliel in the Mishnah, for weestablished a Beraisa (110b) unlike him. This is not a proof. Often, the Gemara often says that a Beraisa is unlike an opinion that the Halachah follows. All the Ge'onim agree that Kesuvah is mid'Rabanan, and we collect 200 Zuz of standard coins, which is an eighth of Tzuri (pure silver) coins. Some say that Kesuvah is mid'Rabanan, but they enacted to pay (Tzuri) money of the Torah, lest it be light in a man's eyes to divorce his wife. The custom is to write '200 Zuz that befit you mid'Oraisa', lest one err to think that it is standard coins. This is reasonable. One who writes '200 Zuz of silver that befit you' does not lose.

Rebuttal (Drishah EH 66:14* DH ul'Fi): If we write 'mid'Oraisa' to show that the coins are Tzuri, we should write the same for a widow! Rather, it means that the Kesuvah is mid'Oraisa.

Defense (Beis Shmuel 14): Chachamim did not enact pure silver for a widow.

Chelkas Mechokek (66:24): The Ramban (110b DH Nosa) holds oppositely. Even R. Shimon ben Gamliel agrees that the Torah did not fix a Shi'ur for the Kesuvah, therefore it is paid with standard coins.

Gra (66:22(17)): In the Yerushalmi, Tana'im argue about whether it is paid with standard or Tzuri coins.

Rambam (Hilchos Ishus 10:7): Chachamim enacted Kesuvah, lest it be light in a man's eyes to divorce his wife.

Hagahos Maimoniyos: Some reverse (the opinions in) Beraisa #1 (10a), so R. Shimon ben Gamliel holds that Kesuvah is mid'Rabanan. This is like the Yerushalmi. Why does he reject the Bavli, which reverses Beraisa #2? Perhaps this is because several Amora'im hold that Kesuvah is mid'Rabanan. They must hold that R. Shimon ben Gamliel's says so in the Mishnah. R. Simchah counsels not to write in the Kesuvah 'that befit you mid'Oraisa', nor to write 'mid'Rabanan'. If it says 'mid'Rabanan'for a Besulah, it is intrinsically counterfeit. This is worse than if no Kesuvah was written!

Ramban (Bereishis 25:6 DH v'Rashi): Rashi says that Pilagshim have (Kidushin, but not) a Kesuvah. This cannot be. Kesuvah is mid'Rabanan! (The Torah distinguishes Pilagshim from wives, so there must be a difference mid'Oraisa!)

Rebuttal (Gra EH 26:7): Rashi holds that Kesuvah is mid'Oraisa. Alternatively, 'Kesuvah' refers to stipulations of the Kesuvah, such as food...

Poskim

Shulchan Aruch (EH 66:6): The Shi'ur Kesuvah is 200 for a Besulah, and 100 for a non-virgin. Both of these are standard coins.

Rema: This is like the Poskim who say that Zuzim of the Kesuvah are mid'Rabanan. Some say that the Shi'ur is in mid'Oraisa Zuzim, which is eight times more than standard coins, and therefore we write 'that befit you mid'Oraisa'. Some say that we write just 'that befit you'. The custom is to write 'that befit you mid'Oraisa' for a Besulah, and 'that befit you' for a widow. This is where there is no custom. Where there is a custom, we follow it.

Beis Shmuel (14): The opinion that requires Zuzim mid'Oraisa agrees that Kesuvah is mid'Rabanan. We hold that a man is believed about Pesach Pasu'ach!