Whom else were the Hebrews permitted to purchase as slaves?
Leviticus 25:45 (ESV)
45You may also buy from among the strangers who sojourn with you and their clans that are with you, who have been born in your land, and they may be your property.
The Hebrews could also have or purchase slaves from non-Hebrews who were resident in Canaan, such as sojourners and temporary residents.
45 You may also buy from among the strangers who sojourn with you and their clans that are with you, who have been born in your land, and they may be your property.