Charles Ellicott 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)

If a man sell his daughter to be a maidservant. —The right of selling their children into slavery was regarded in ancient times as inherent in the patria potestas, and was widely practiced by many nations (Herodotus v. 6; Heyne, Opusc., vol. iv., p. 125). Among the Hebrews such sales were, comparatively speaking, rare; but still they occasionally took place, in consequence of extreme poverty (Nehemiah 5:5). Women sold in this way might claim their freedom at the end of six years if they chose (Deuteronomy 15:17); but if purchased to be wives, they received a further protection. If the intention were carried out, they were to be entitled to the status of wives during their whole lifetime, even though their husbands contracted further marriages (Exodus 21:10).

If, instead of becoming the wife of her purchaser, a woman was given by him to his son, she was to enjoy all the rights of a daughter (Exodus 21:9). If the purchaser declined to act in either of these two ways, he was compelled to take one of two other courses. Either he had to arrange for another Hebrew to discharge his marriage obligation (Exodus 21:8), or he had to return the young woman unharmed to her father, without demanding the return of the purchase money (Exodus 21:11). These provisions afforded considerable protection to the slave-concubine, who might otherwise have been liable to severe wrong and oppression.