A female slave was treated differently from a male slave. She may have been sold with the expectation that she would become a concubine (a secondary wife). Releasing her after seven years would not necessarily protect her.
If she were sent away, she might have to return to her father. Her father had already sold her, which suggests he did not want her. Marriage would also be unlikely for a former concubine. She might even be forced into prostitution. For this reason, it was often considered better for her to remain in her master’s household.
Sometimes a man bought a girl to become his son’s wife (21:9). In that case, the payment was not the price of a slave. It was similar to a bride-price (a gift given to a bride’s family at marriage). In the ancient Near East, a young man usually gave money or valuable gifts to the bride’s father (see 22:16; Genesis 24:53). A woman for whom a bride-price had been paid was not considered a slave.