Gemara

38a (Mishnah): If she does Yibum she is like his wife in all respects, but the Kesuvah is paid from the property of the deceased.

39a - Question: What is the reason?

Answer: She was acquired to him by Shomayim.

If the Mes did not leave enough property to pay a Kesuvah, she has a Kesuvah from the Yavam's property, so that it will not be light in his eyes to divorce her.

85a (Mishnah): Sheniyos mid'Rabanan (do not receive a Kesuvah...)

Question (Benei Biri): If a woman is a Shniyah to her husband but not to her Yavam, does she receive a Kesuvah?

Since we learned that a Yevamah collects her Kesuvah from the Mes' property, she does not have one;

Or, since we enacted a Kesuvah from the Yavam when there is no property from the Mes, here also!

Answer (Rav Sheshes - Beraisa): A Yevamah collects her Kesuvah from the Mes' property. If he left no property, they enacted a Kesuvah from the Yavam. If she was a Shniyah to the Mes, she does not have a Kesuvah from the Yavam.

Rishonim

Rif and Rosh (12b and 4:14): A Yevamah who did Yibum collects her Kesuvah from the Mes' property. If he did not leave enough property to pay a Kesuvah, she has a Kesuvah from the Yavam's property, so that it will not be light in his eyes to divorce her.

Ran (Kesuvos 40b DH Masnisin): The Gemara asked what is the reason, i.e. that she does not receive a second Kesuvah from the Yavam. It answered that Shomayim acquired her to the Yavam, therefore he need not give her anything.

Question (Tosfos Yevamos 39a DH Ishah): What difference does it make whether or not the lien for her Kesuvah is on the Yavam's property? Even if it is on the Mes' property, the Yavam inherited it! If we say that she collects from property that the Mes sold even though the Yavam himself has property, it will be light in his eyes to divorce her!

Answer #1 (Tosfos): If the Mes' property was flooded after he divorced her, she cannot collect from the Yavam's property.

Answer #2 (Ran): If the Yavam sold his own property and the Mes' property was flooded, she cannot collect from what he sold. Alternatively, only the Ikar Kesuvah is collected from the Yavam, but not her dowry, even if it cannot be collected from the Mes' property, and even if the Yavam spent enough of the Mes' property to repay the dowry.

Chazon Ish (EH 135: Perek ha'Ishah she'Noflu 3): Surely, the Mes was obligated to return the dowry she brought to him, so it cannot be collected from the Yavam's property! Perhaps one might have thought that it is as if she already collected it and brought it to the Yavam for a dowry. Alternatively, even if there are intact remnants of her dowry (after she is widowed or divorced from the Yavam) she gets them back only from the Mes' property.

Answer #3 (Darchei Moshe EH 168:1): The Ran (Kesuvos 20a DH Bas) says that the Yavam's daughters from the Yevamah are fed after his death only when the Kesuvah is from his property.

Answer #4 (Tosfos Kesuvos 81a DH ha'Rotzeh): If the Mes sold enough property to pay the Kesuvah, she collects from the buyers, not from the Yavam.

Question (Beis Yosef EH 168 DH v'Rabeinu): The Tur says like this. Why are we not concerned lest it be light in his eyes to divorce her?

Answer #1 (Taz 168:1): The Yavam is concerned lest the sold property get flooded or taken by extortionists, and she will collect from him.

Question (Chazon Ish ibid. 2): If the property was not flooded or taken, it is light for him to divorce now (he is exempt if it is flooded later)! Perhaps he avoids quarreling with her lest the property be flooded before he divorces her.

Answer #2 (Bach Sof Siman 168): Regarding a Yevamah there is no concern lest it be light in his eyes to divorce her.

Note: The Bach does not explain why. Perhaps a man appreciates the greatness of Yibum. It overrides Ervah, and the Torah commands to spit at and insult one who declines the Mitzvah. When the Bach discusses a Shniyah to the Mes (brought below), he says that the lack of a Kesuvah helps deter divorce. Perhaps one who receives a 'free' wife does not squander his good fortune, for the chance that this would happen again is minuscule.

Rif and Rosh (Yevamos 27a and 9:2): If she was a Shniyah to her husband, she does not have a Kesuvah from the Yavam.

Nimukei Yosef (ibid.): Our Gemara did not ask about when she was permitted to her husband and is a Shniyah to the Yavam. It holds that since she did not transgress, surely she has a Kesuvah. The Yerushalmi asked this and did not settle it. According to the Yerushalmi she would not get a Kesuvah. However, we rely on the BAvid l'Igluyeii, which holds that she would get one.

Rambam (22:11): The Kesuvah of a Yevamah is collected from the property of the deceased.

Rambam (14): If a Yevamah did not have a Kesuvah from her husband or she pardoned it, the Yavam acquired his brother's property and he may sell or give it like he desires. When he marries her he writes her a Kesuvah of 100, and there is a lien on all his property to pay it, like any other woman with a Kesuvah.

Rambam (Hilchos Yibum 2:17): If a Yevamah did not have a Kesuvah from her husband because she was forbidden to him, and she is permitted to her Yavam, he may do Yibum. She has no Kesuvah, just like she had no Kesuvah from her husband. The law of Tosefes Kesuvah with the Yavam is like it was with her husband. If her first husband (was permitted to her but) did not write a Kesuvah, or she sold or pardoned it, the Yavam must write her a Kesuvah, like other widows.

Question: Before Yibum, surely they did not obligate him to give a Kesuvah. After Yibum, surely he may not keep her without a Kesuvah. This is like Bi'as Zenus!

Answer (Magid Mishneh and Nimukei Yosef 27a DH Gemara): The Gemara asked about one who did Yibum and wants to divorce her. Indeed, he may not keep her (without writing a Kesuvah) because it is Bi'as Zenus.

Lechem Mishneh: The Magid Mishneh's reasoning is solid, but the Rambam connotes that he may keep her without a Kesuvah. This requires investigation.

Bach (EH 168 DH v'Chosav Od): The Gemara says that Chachamim did not enact a Kesuvah when she was a Shniyah to her husband. The Rambam infers that if she is not to blame for not having a Kesuvah from the Mes, she receives one from the Yavam. The Tur cites the Rambam to say that if she was a Shniyah and is permitted to the Yavam and does Yibum, she has no Kesuvah. I.e., he may keep her without a Kesuvah, unlike the Magid Mishneh. Since she had no Kesuvah from her husband and Shomayim acquired her to the Yavam, it will not be light in his eyes to divorce her. Alternatively, we are not concerned if it is light in his eyes to divorce a woman who married (his brother) b'Isur.

Poskim

Shulchan Aruch (EH 168:3): The Kesuvah of a Yevamah is collected from the property of the deceased.

Beis Shmuel (EH 168:2): Tosfos (Yevamos 39a) says that if the Mes' property was flooded after he divorced her, she cannot collect from the Yavam's property. This implies that if it was flooded before divorce, she collects. The Tur's text of a Kesuvas Yevamim implies that the Yavam accepts respectively only to feed her. If so, in this case he must write a new Kesuvah.

Note: The Bach (ibid.) says that the Tur holds like Tosfos in Kesuvos, that we are not concerned if a Yevamah has no Kesuvah.

Shulchan Aruch (8): If a Yevamah did not have a Kesuvah from her husband or she pardoned it, the Yavam writes to her a Kesuvah of 100 when he does Yibum. There is a lien on all his property to pay it, like any other woman with a Kesuvah.

Beis Shmuel (6): Even if he did not write it she has a Kesuvah due to Chachamim's enactment.

Beis Yosef (DH u'Mah she'Omar Rabeinu): The Rambam and Tur say that when the Yavam writes for her a Kesuvah, it is for 100. This is obvious.

Chazon Ish (ibid. 3): The Ran (Kesuvos 40b, brought above) says that in this case she collects only Ikar Kesuvah from the Yavam, but not her dowry. Perhaps he holds that she gets 200.

Shitah Mekubetzes (Kesuvos 82b): Talmidei R. Yonah say that in this case she gets 100 or 200 depending on whether she fell to Yibum from Nisu'in or Eirusin. The Rivash cites an opinion that she gets 200; Chachamim enacted that she gets whatever she had from the Mes.

Shulchan Aruch (9): If a Yevamah did not have a Kesuvah from her husband because she was forbidden to him and she is permitted to her Yavam, he may do Yibum. She has no Kesuvah, just like she had no Kesuvah from her husband.

Rema: This is only if he did not write for her a Kesuvah. He may not keep her without a Kesuvah.

Shulchan Aruch (ibid.): The law of Tosefes Kesuvah with the Yavam is like it was with her husband. If her first husband (was permitted to her but) did not write a Kesuvah, or she sold or pardoned it, the Yavam must write her a Kesuvah, like other widows.

Rema: If there is no property from the first husband the Yavam writes for her a Kesuvah (of 100) like a widow.