What is the ruling if a man divorces a slave woman he has had intercourse with, and then regains ownership of her before having intercourse with her sister?

Primary text

If the man divorces the woman he had intercourse with, and ownership returns to him before he has intercourse with her sister, the first woman is lawful for him to have intercourse with, but her sister is prohibited to him because the sister is currently his conjugal property (*firash*).