I don't know if this was used as justification for these practices, but see Deuteronomy 15:12-13 (https://www.biblegateway.com/passage/?search=Deuteronomy+15&...): "If a member of your community, whether a Hebrew man or a Hebrew woman, is sold to you and works for you six years, in the seventh year you shall set that person free. 13 And when you send a male slave out from you a free person, you shall not send him out empty-handed." Note that you only have to help out slaves upon freeing them if they are Hebrew, i. e. one of your own people.