John Gill Commentary Exodus 21:9

"And if he espouse her unto his son, he shall deal with her after the manner of daughters." — Exodus 21:9 (ASV)

And if he have betrothed her to his son
Not caring to betroth and marry her himself, as being more suitable in age for his son than for himself;

he shall deal with her after the manner of daughters ;
as if she was his daughter, and give her a dowry: or the son shall treat her after the manner the daughters of Israel are treated when married, by giving her food, raiment, and the duty of marriage, so Jarchi: or after the manner of the daughters of Israel that are virgins, and who are not sold, as Aben Ezra.