Kesubos Chapter 8, Mishnah 6. This Mishnah deals with a shomeres yavam - a woman whose husband died without children so that she is bound to yibum, and who has not yet undergone either yibum or chalitzah - and with the status of property that came to her by inheritance during that period.
The text of the Mishnah:
"Shomeres yavam shenaflu lah nechasim - modim Beis Shammai u'Beis Hillel shemocheres v'nosenes v'kayam" - a shomeres yavam who inherited property: both academies agree that she may sell the property or give it away as a gift, and the sale or the gift is valid.
This ruling seems to stand in contrast to the first Mishnah of our chapter. There we learned about a betrothed woman who was married through kiddushin but had not yet entered the marriage, and property came to her during the period of betrothal, and Beis Shammai and Beis Hillel disagreed whether she may sell it in the first place, with Beis Hillel holding that she may not sell. Here, however, in the case of a shomeres yavam, even Beis Hillel agree that she may sell it in the first place.
The reason is that the yevamah is not bound to the yavam the way a betrothed woman is bound to the man who betrothed her. Once the betrothal has taken place, the couple will presumably go on to the chuppah, and it is only that this has not yet happened in practice. With a yevamah, however, nothing is known: perhaps there will be chalitzah, and perhaps they will never come to marriage at all. The matter is in doubt, and therefore she may sell in the first place.
"Meisah - mah ya'asu bikesubasah u'vanechasim hanichnasin v'yotz'in imah":
What is the law if she died while still a shomeres yavam? The Mishnah discusses two categories of money:
Her kesubah - this includes the basic amount of the kesubah (two hundred if she married as a virgin, one hundred if she married as a widow), the additional sum the husband added to it from his own funds, and nichsei tzon barzel - the dowry recorded in the kesubah document, for which the husband accepted responsibility and undertook to pay its value whenever the marriage should end.
"Nechasim hanichnasin v'yotz'in imah" - these are nichsei melog, which remain in her own ownership.
The dispute between Beis Shammai and Beis Hillel:
Beis Shammai say: "Yachloku yorshei ha'av im yorshei haba'al" - her father's heirs, meaning her own family (her father, and if he is no longer alive, her brothers), and the heirs of the deceased husband, who are his brothers, divide everything equally.
Beis Hillel say that it depends on the category of property:
The property recorded in the kesubah (nichsei tzon barzel) remains in the possession of both sides - the father's heirs and the husband's heirs - since both have a connection to it: on the one hand she brought it in from her father's house, and on the other hand it is tied to the husband's family, since he accepted responsibility for it, and therefore they divide it between them.
The kesubah itself, that is, the basic amount of the kesubah - one hundred or two hundred - together with any additional sum he added for her, remains in the possession of the husband's heirs, since it was the husband who bore the responsibility to pay the kesubah.
The property that was hers, the nichsei melog, remains in the possession of the father's heirs, her relatives who inherit from her, since it is her own property.
In summary: in this Mishnah we learned that in the case of a shomeres yavam, Beis Shammai and Beis Hillel agree that property that came to her may be sold or given away and the transaction is valid, unlike the case of a betrothed woman, because there is no certainty that the bond will lead to marriage. And if she died, they disagreed: according to Beis Shammai the father's heirs and the husband's heirs divide everything; according to Beis Hillel, the nichsei tzon barzel remain in the possession of both, the kesubah and its addition in the possession of the husband's heirs, and the nichsei melog in the possession of the father's heirs.