Adam Clarke on Exodus 21:7

If a man sell his daughter - This the Jews allowed no man to do but in extreme distress - when he had no goods, either movable or immovable left, even to the clothes on his back; and he had this permission only while she was unmarriageable. It may appear at first view strange that such a law should have been given; but let it be remembered, that this servitude could extend, at the utmost, only to six years; and that it was nearly the same as in some cases of apprenticeship among us, where the parents bind the child for seven years, and have from the master so much per week during that period.