Exodus 21:4

If his master give him a wife, an handmaid, and she bear him sons or daughters, the wife and her children shall belong to his master, and he may go out alone.

IF HIS MASTER GIVE HIM A WIFE. “Scripture is speaking of a Canaanite woman. Or perhaps this is not so; but Scripture here speaks only of an Israelite woman?! Scripture therefore says, the wife and her children shall be her master’s. Consequently, it must be speaking of a Canaanite woman.” This is the language of the Beraitha, Note 209. taught in the Mechilta. Now Rashi wrote [in explanation of this Mechilta]: “For a Hebrew maidservant also goes free at the end of six years [just as a Hebrew manservant does], or even before the end of six years if she shows signs of puberty, for it is said, If thy brother, a Hebrew man, or a Hebrew woman, be sold unto thee, he shall serve thee six years.” can only be speaking of a Canaanite woman.
But this is not quite correct. For if we say [as the Beraitha above attempted to,] that the verse here speaks of an Israelite woman, it could no longer refer to the case of a father selling his minor daughter, about whom the law is given that when she shows signs of puberty she goes free, for how could the master give her as a wife to his Hebrew servant, since he has no power to hand her over to any other man [except to designate her to be his own wife, or that of his son — as is explained further in Verses 8-9]! who sold herself as a maidservant. So how then could Rashi support his argument [that if she were an Israelite woman, she would go free “if she shows signs of puberty],” when the Beraitha must of necessity be referring not to a minor but to a woman of age? (Mizrachi). Similarly, the proof that Rashi mentioned, namely that she also goes free at the end of six years, is only so in accordance with his own words which he wrote that a person who sells himself [on account of his destitution] is sold for a maximum of six years; but in the Talmud these are the words of a single Sage [Rabbi Eliezer], but the accepted opinion is that one who sells himself can be sold for six years or more. Now if so, the case of a woman who goes free at the end of six years can only be when her father sold her [as a minor, but in that case the master has no right to give her as a wife to his Hebrew servant, but only to designate her as his own wife or that of his son]!
But that which the Rabbis have said [in the Beraitha above, on the basis of the verse, the wife and her children shall be the master’s]: “Consequently, Scripture must be speaking of a Canaanite woman” — the meaning thereof is as follows: Since He stated, the wife and her children shall be the master’s [it must be speaking only of a Canaanite woman], for the children of a Canaanite bondmaid are the master’s since her child has the same status as she does, but in the case of an Israelite woman — even if she were of age [in which case her master could give her to his Hebrew servant as a wife], and even if we were to say that a woman may sell herself as a maidservant (Verse 7) — a man can sell himself, but a woman cannot sell herself.” But Ramban is writing only on the assumption that the Beraitha introduced this as a hypothesis finally to be disproved. See my Hebrew commentary pp. 515-516. — her children are the father’s [not the master’s].

אם אדוניו, “if his master etc.” here the Torah authorises the master of this Hebrew (unmarried) servant to give him one of his Canaanite slave-women as a “wife” to have children with, provided he had previously been married to a Jewish wife. If this “servant” had previously not been married, he cannot force him to enter into such a relationship with a Canaanite woman. This ruling is based on the words in verse 3: אם בגפו יבא בגפו יצא, ”if he came as a bachelor, he will leave as a bachelor.” The reason underlying this legislation is that if he had previously been married to a Jewish woman he will not feel attracted to a Canaanite slave woman.