Yevamot 39A

וקסבר אביי ידו כידה.

And Abaye holds that according to Beit Hillel, with regard to the husband’s rights to his wife’s property, his hand is like her hand, but not any stronger. Accordingly, the yavam has a weaker hand in her property than she does, because the rights of a yavam are always weaker than those of the husband himself. Therefore, she is considered to be in possession of the property, and when she dies, her heirs, i.e., her father and his heirs, inherit it. However, Beit Shammai assume that the husband’s hand is stronger than his wife’s hand. Therefore, the hand of the yavam, which is weaker than the husband’s hand, is nevertheless equally as strong as the hand of the yevama, and therefore they rule that if she dies, the yavam and her heirs divide up the property.

אמר ליה רבא: אי דנפלי לה כשהיא תחתיו דבעל ⁠— דכולי עלמא לא פליגי דידו עדיפא מידה.

Rava said to him: I hold that if the property was bequeathed to her when she was still under the first husband, everyone, i.e., both Beit Hillel and Beit Shammai, agrees that his hand is stronger than her hand, and therefore the hand of the yavam will be equally as strong as that of the yevama, and if she dies the property will be divided between the two sides.

אלא אידי ואידי דנפלו לה כשהיא שומרת יבם. רישא דלא עבד בה מאמר, סיפא דעבד בה מאמר.

Therefore, Rava presents his own resolution to the apparent inconsistency in Beit Shammai’s rulings in the Mishnah: Rather, both this first clause and that latter clause concern cases in which property was bequeathed to her when she was a widow waiting for her yavam. The first clause concerns a case in which the yavam had not performed a levirate betrothal with her, and the latter clause concerns a case in which he had performed a levirate betrothal with her.