Or HaChaim on Exodus 21:9/אור החיים על שמות כ״א:ט׳

1 ואם לבנו ייעדנה, And if he espouse her to his son, etc. This means that the master never espoused her to himself, but intended her as a wife for his son when he bought her. The Torah informs us that this is perfectly in order, i.e. the money the father had paid to the girl's father for her is acceptable as the money for the betrothal to his son.

2The words כמשפט הבנות יעשה לה, he shall deal with her after the manner of daughters, apply to both the father (master) and to his son. The verse following will spell out exactly what these obligations are that the master/son assumes when purchasing such a maidservant.