Reference for Bekhorot 66:2

דילמא עד כאן לא קאמר ר' אלעזר בר' שמעון התם אלא פסולי המוקדשין דאלימי למיתפס פדיונו אבל בכור דלא אלים למיתפס פדיונו לא

- Perhaps R'Eleazar son of R'Simeon holds that it is forbidden only in the case of sacrifices which have become unfit, for they are competent to be redeemed, but in the case of a firstling which is not competent to be redeemed, it is different.<span class="x" onmousemove="('comment',' And therefore one may flay the skin of a firstling from its feet.');">²</span>

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