Deuteronomy 21:14

And it shall be, if thou have no delight in her, then thou shalt let her go whither she will; but thou shalt not sell her at all for money, thou shalt not make merchandise of her, because thou hast humbled her.

Cross-references

Deuteronomy 22:29 forces a rapist to marry his victim and never divorce, contrasting with this law that allows release of a displeasing captive.

Exodus 21:7-11 gives similar legal protections for a female slave taken as wife—she must be freed if not treated properly, not sold.