Chabad.org — English translation + Rashi
Torah Temimah points to Toras Kohanim (Sifra), Vayikra Dibbura DeChovah, on 4:12 (ve-saraf oto — the valid, not the invalid) (unit 4; single primary locus per emergency Core plan).
Sefaria — Sifra / Toras Kohanim, Vayikra Dibbura DeChovah
English: “And he shall burn it” — the valid, and not the invalid.
Only a valid bull is burned outside the three camps — if the bull was invalidated, its burning is not governed by this outside-the-camps rule.
The special procedure of burning outside the three camps applies solely to a kasher chatas bull. Once the animal has been declared pasul, the verse’s “ve-saraf oto” no longer dictates the place or manner of its disposal. Validity is a precondition for the entire outside-burning regimen.