Rashi on Leviticus
ועבדך ואמתך אשר יהיו לך BUT THY MAN-SERVANT AND THY MAID-SERVANT WHO MAY BE THINE [SHALL BE OF THE NATIONS THAT ARE ROUND ABOUT YOU] — If you should say, “But if this be so (that I must treat my Jewish servant with so much regard), by whom shall I be served? Over my Jewish servants I have no power; of the Canaanite people I cannot possess any for You have warned me, (Deuteronomy XX 16) “Thou shalt keep alive any soul"! Who then shall serve me?” Then I reply —
Siftei Chakhamim
If you say: “If so, etc.” It seems that the verse is the source of this question. I.e., [the verse itself is hinting at the question], “If you say, etc.” Because if not so [that the verse is hinting at question], why does it say “And your slaves (ועבדך)” with a vav, connecting it to the above. Why do we need this? Therefore Rashi writes that there is a question here, and answers it.
Haamek Davar on Leviticus
From them you may purchase. It is a mitzvah to purchase them to take them out of idolatry.