Reference for Chullin 281:27

כגון שנטלה על מנת לשלח דלאו ליכא עשה הוא דאיכא וליתי עשה ולידחי עשה

then so long as this man has not slaughtered the dam he has not transgressed the negative precept.<span class="x" onmousemove="('comment',' It cannot therefore be suggested that the positive precept of the leper's sacrifice should override the law of letting the dam go for the latter still involves a positive and a negative precept; accordingly the verse stated above to exclude this is now superfluous.');">¹⁹</span>

Rashi on Deuteronomy

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