Yevamot 21A

באלמנה מן הנשואין ⁠— כולי עלמא לא פליגי דלא פטרה, דאין עשה דוחה לא תעשה ועשה. כי פליגי באלמנה מן הארוסין. מאן דאמר פוטרת ⁠— אתי עשה ודחי את לא תעשה. ומאן דאמר אינה פוטרת ⁠— לא אתי עשה ודחי את לא תעשה, כיון דאפשר בחליצה.

As for a widow from marriage, everyone agrees that this does not exempt the rival wife, as a positive mitzva does not override a prohibition and a positive mitzva together. When they disagree, it is in the case of a widow from betrothal. The one who said that it exempts the rival wife holds that the positive mitzva of levirate marriage comes and overrides the prohibition against a High Priest marrying a widow. And the one who said that it does not exempt the rival wife held that a positive mitzva does not come and override a prohibition, since it is possible to perform ḥalitza.

מיתיבי: ואם בעלו קנו! תיובתא.

The Talmud raises an objection to the opinion that the rival wife would not be exempt, from the previously cited baraita: If one of those yevamin had sex, he acquired her as a wife. This shows that the act of levirate marriage is valid, even when prohibited. If so, in the case of the High Priest and a widow as well, her rival wife should be exempt. This is a conclusive refutation, and that opinion is rejected.

לימא תיהוי נמי תיובתא דריש לקיש?! אמר לך ריש לקיש: כי אמינא אנא ⁠— היכא דמקיימי מצוה, אבל הכא, חליצה במקום ייבום ⁠— לאו מצוה היא.

The Talmud asks: should we say that this is also a conclusive refutation of the opinion of Reish Lakish and therefore reject his principle? The Talmud answers: Reish Lakish could have said to you: When I say that in every place that you find a positive mitzva and a prohibition applying to the same matter, if you can fulfill both of them together without transgressing the prohibition, this is best, and the positive mitzva does not override the prohibition, this is applicable only when the mitzva is fulfilled properly; but here, ḥalitza in place of levirate marriage is not considered a proper fulfillment of the mitzva.