Seeing as we are obligated to kill all members of the seven Cana'ani nations and we are not permitted to subjugate a fellow-Jew, who will perform the tasks that one may not command an Eved Ivri to do?
Rashi: Hence the following Pasuk permits making use of the surrounding nations, who are not Cana'anim, and whom one is therefore permitted to purchase as slaves.
Why does the Torah insert the (otherwise superfluous) words "asher Yih'yu lach"?
Sifra: To teach us that if a Yisrael has relations with his Shifchah Cana'anis and she bears him a son, he is allowed to take him as a slave.
What are the implications of "asher Sevivoseichem"?
Sifra: It implies that one is not permitted to purchase Avadim from Eretz Cana'an. 1
See Torah Temimah, note 225.
What do we learn from the (otherwise superfluos) word "Meihem Tiknu Eved ve'Amah"?
Moshav Zekenim citing Gitin 38a: You can purchase Nochrim (Kinyan ha'Guf - they become your property), but they cannot purchase you (they can only acquire an Eved Ivri for his labor), or one another. 1
Sifra: To teach us that one may purchase not only their children as slaves, but also them.
Oznayim la'Torah: The Torah writes "Meihem Tiknu Eved ve'Amah" and not 'bahem Ta'avdu be'Farech' to teach us that, as opposed to other sales, it is preferable to buy an Eved Cana'ani from Nochrim than from a Yisrael. 2
See Oznayim la'Torah, who gives two reasons for this