Exodus 21:8 "He shall have no power to sell her"

A young Jewish girl who is sold into slavery is called an "Amah Iv'riah".

The Torah instructs her master to treat her with respect and tenderness. He is encouraged to marry her or give her as a bride to his son (see Positive Mitzvah 233).

If the master or his son does not marry her by the end of her service, he is forbidden to sell her as a slave to anyone else and must grant her freedom.