Halakhah su Bekhorot 7:8

Finally, the Mishnah (Bechorot 7:6) discusses a bechor (firstborn) who had an extra finger and removed it, but the Mishnah does not comment on his right to do so. In contrast, earlier Mishnayot (Bechorot 1:1 and 2:1), discussing one who sells his cow to a nochri (gentile), do indeed criticize the sale, adding, “even though one does not enjoy the right to do so.” Since the Mishnah does not condemn removing an extra finger, we may infer that it permits doing so.