Charles Ellicott Commentary Deuteronomy 15:17

"then thou shalt take an awl, and thrust it through his ear unto the door, and he shall be thy servant for ever. And also unto thy maid-servant thou shalt do likewise." — Deuteronomy 15:17 (ASV)

And to your maidservant you shall do likewise —i.e., “in furnishing her liberally” (Rashi), and “possibly also in retaining her if she wishes.”

It must not be supposed that this contradicts Exodus 21:7, “She shall not go out as the menservants do.” She shall not go out according to the going of the male servants (i.e., on the same principle). It is not said, She shall not go out at all. The exceptions are given in Exodus 21:8-11 (see these verses). The general right of release is stated here. One difference (as stated by Rashi) is that women were not liable to be sold for theft like men, but might be sold by their parents in infancy.

If the girl were not marriageable when the first Sabbatical year arrived, she would obtain her freedom absolutely, because the case contemplated in Exodus 21:8-10 could not possibly arise. And, generally, we may suppose that the rights of an unmarried female slave would be the same as those of a man, to go out free in the seventh year .