If she is not pleasing to the master, who should have betrothed her… (Exodus 21:8)
The rules for a female servant are completely different from those for a male. A man, upon undergoing extreme hardship, might “sell” his minor daughter into marriage so that she can have a better life. The “buyer” must betroth the girl to himself or to his son. In such a case, she must be treated no differently than any other wife. If taking another wife mean’s that this girl’s standard of living will be reduced, the man is not allowed to marry another woman in addition to her (see Mitzvah #46).
There is an alternative to marrying the girl: arranging for her to be redeemed, as we will see in the next mitzvah. However, while the man has the option to redeem her, he’s SUPPOSED to marry her (see Mishna Bechoros 1:7). The Torah doesn’t come out and say “he must betroth the Jewish maidservant,” but it does say, “whom he should have betrothed,” which makes it equally clear that such was his obligation.
Actually, our verse includes a kri/k’siv, in which the Torah text is read one way and written another. The verse says, “asher lo y’adah.” The text says the word “lo” with a Hebrew letter alef, meaning “He did not betroth her.” The reading is done as if “lo” were written with the letter vav, meaning, “It was upon him to betroth her.” The double meaning is that it was this man’s responsibility to see to this girl’s wedding and he blew it.
This mitzvah continues the theme of compassion, as was at the heart of the mitzvah regarding the male servant. Misfortune drove this family to such extremes and G-d commands us to treat them kindly. Rather than making a servant of the girl, her master is supposed to marry her and make her the lady of the household (or designate her for his son).
It should be obvious that this mitzvah only applies to men. It also only applies at a time when the Jubilee year is observed. It is discussed in the Talmudic tractate of Kiddushin starting on page 18. (The differences between male and female servants starts on 18a; the obligation to betroth the female starts on 18b.) In the Mishneh Torah, it is found in Hilchos Avadim chapter 4. It is #233 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.