TT #743 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:09
Then his yevamah shall approach him in the presence of the elders, and draw his shoe off his foot, and spit in his face, and respond and say: “Thus shall be done to the man who will not build up his brother’s house.”
Torah Temimah points to Yevamot 24a.
TT lemma: אֲשֶׁר לֹא יִבְנֶה אֶת בֵּית אָחִיו.
אֲשֶׁר לֹא יִבְנֶה – וְלֹא אֲשֶׁר לֹא בָּנָה; מִכָּאן שֶׁאִם כָּנַס וּגֵרֵשׁ – אֵין חוֹלְצִין.
English: “Who will not build” — and not “who has not built”; from here that if he took her in yibbum and then divorced her, chalitzah is not performed.
On the daf / in context: Once yibbum has been performed, the subsequent dissolution of the marriage does not reopen the possibility of chalitzah.
Derash from the future tense.
If yibbum was performed and the couple later divorced, no chalitzah is required or possible.