Sforno on Exodus 21:7/ספורנו על שמות כ״א:ז׳

1לא תצא כצאת העבדים. It is not appropriate for an upstanding member of society to buy a Jewish girl as a servant against her will. Such a “sale” is acceptable only if the girl in question will become the wife of the buyer or his son when reaching puberty. The purchase price will be given to her father as our sages have stipulated in Ketuvot 46.