Deuteronomy 21:14 "You shall not sell her for money"

This Negative Mitzvah applies to a specific situation which may occur during wartime.

If a Jewish soldier sees a woman among the captives of a non- Jewish city and wants to marry her, he must follow the Torah's instructions for doing so (see Positive Mitzvah 221).

If he later decides not to marry her, he is forbidden to sell her as a servant.