Rashi on Leviticus 25:45/רש"י על ויקרא כ״ה:מ״ה

1‎ וגם מבני התושבים MORE-OVER FROM AMONGST THE TOLERATED STRANGERS who came from the lands round about you in order to marry Canaanite women in your land and these bear them children; then the child follows the father in respect of nationality, and thus, not being regarded as a Canaanite, is not included in this command, “Thou shalt not keep alive any soul”, but you may purchase him as a servant (cf. Sifra, Behar, Section 6 4; Kiddushin 67b).

2מהם תקנו means THEM MAY YE BUY.