פירוש על קידושין 49:4

R. Hisda tells R. Hamnuna that he could have answered the question from a mishnah. The mishnah refers to a person who has a skin affliction. If there is a healthy, unaffected spot within the affliction, then he is rendered impure. The mishnah lists parts of the body which if unaffected do not count as a healthy spot and do not render him impure. On this mishnah a baraita is taught according to which a slave goes free if his master cuts off these parts (ouch). Since Rabbi includes castration we have the answer to our question. A slave castrated by his master goes free.