I don’t know if this will help but a couple commentaries touch upon this topic…
In the 1859 Haydock Catholic Commentary on Exodus 21:7 it says:Ver. 7. Go out, to work in the fields, according to Grotius; or rather, to enjoy her liberty. A father who sold his daughter, always expected that she should be the wife of the purchaser, or his son. If this did not take place, she was free after six years, or before, if her master died. Constantine sanctioned the power of the Romans to sell their children. The Phrygians and Thebans had the like custom. (Calmet)
In A Catholic Commentary on Holy Scripture, ed. by Bernard Orchard, pub. by Thomas Nelson & Sons, 1953, on Exodus 21:7-11, it says in part:7-11. The case of a maiden sold into slavery to become the slave-wife of her master is considered. If she displease her master who has determined her for himself, he may not sell her to foreigners but must either let her be bought back by her father or, if he give her in marriage to his son, treat her as a daughter, or ‘if he take another slave-wife, her food, her raiment, her marriage intercourse he shall not diminish’. Any infraction of these three rights sets her free.