פירוש על קידושין 31:8

Since R. Huna holds that the master writes the document, he can derive that a female slave is acquired by a document without using verse “she shall not go out as male slaves do.” Therefore, he can use this verse to teach that she does not go free if her master causes her to lose a limb. Only non-Jewish slaves go free under these circumstances.

R. Hisda can learn both rules—that she is acquired as are non-Jewish slaves and that she does not go free at loss of limb as do male slaves—from this one verse since the verse has an extra word--“go out.”