That the one who acquires the Hebrew bondwoman from the father not sell her: That anyone who acquires a Hebrew bondwoman not sell her to another man ever, as it is stated (Exodus 21:8), “he shall not have the right to sell her to a foreign people, etc.” And the explanation [of the phrase] is like [Onkelos’ Aramaic] translation, “to another man.” And it was stated with this wording to distance this thing — meaning to say that if he sells her to a second man, it is for this poor little one similar to if he sold her to a foreign people.

From the roots of this commandment is that God wanted to give us merit and commanded us to act with the trait of compassion, which is beloved in front of Him.

The laws of the commandment are written above in the commandment of designation (Sefer HaChinukh 43). (See Mishneh Torah, Laws of Slaves 4.)