Yevamot 107B

ומציא עקרא! אלא: ביאה ומאמר, דהוא קעביד ⁠— מציא עקרא. זיקה, דרחמנא רמא עלה ⁠— לא מציא עקרא. עולא אמר: ממאנת אף לזיקתו. מאי טעמא ⁠— נישואי קמאי קא עקרא.

and she can nullify it, as she can subsequently refuse the yavam with whom she entered into levirate marriage. Rather, the reasoning is: With regard to consummation of the levirate marriage and to levirate betrothal, both of which he performs, she can nullify them. But with regard to the levirate bond, which God imposes upon her at the death of her first husband, she cannot nullify it. Whereas Ulla said: She may direct her refusal even to his levirate bond. What is the reason? By refusing, she nullifies the original marriage, rather than the levirate bond that resulted from the death of her husband.

איתיביה רבא לעולא: כל שיכולה למאן ולא מיאנה ⁠— צרתה חולצת ולא מתייבמת. ואמאי? תמאן השתא ותעקרינהו לנישואי קמאי, ותתייבם צרתה! צרת ערוה שאני. דתני רמי בר יחזקאל: מיאנה בבעל ⁠— מותרת לאביו. ביבם ⁠— אסורה לאביו.

Rava raised an objection to the statement made by Ulla. It is taught in a baraita: In any case of levirate marriage in which a minor girl is forbidden to the yavam and she could perform refusal but does not perform refusal, her rival wife performs ḥalitza and may not enter into levirate marriage. Rava continues: Why? Let her perform refusal now and nullify the first marriage retroactively, so that the so-called rival wife was never really a rival wife of a forbidden relative at all, and let her rival wife enter into levirate marriage. The Talmud answers: A rival wife of a forbidden relative is different, due to a rabbinic decree, as Rami bar Yeḥezkel taught in a baraita: A minor girl who refuses her husband is permitted to his father, because the marriage was annulled and she is no longer his daughter-in-law. But one who refuses a yavam is forbidden to his father.