Albert Barnes Commentary Exodus 21:7

"And if a man sell his daughter to be a maid-servant, she shall not go out as the men-servants do." — Exodus 21:7 (ASV)

A man might, in accordance with existing custom, sell his daughter to another man, intending for her to become an inferior wife or concubine. In this case, she was not “to go out,” like the bondsman; that is, she was not to be dismissed at the end of the sixth year. But women who were bound in any other way, seem to have been under the same conditions as bondsmen. See Deuteronomy 15:17.