A Hebrew master of a non-Hebrew slave ought to be working to turn that slave into a Hebrew. If a Hebrew owns a non-Hebrew slave more than seven years, then he's not doing his job as a master. (Lev 25-26, Exo 21, Jer 34)

A Hebrew master knows he's done well when his foreign slave exhibits 3 behaviors:

1) Worships YHVH instead of false gods
2) Keeps the Sabbath
3) Reverences the Temple and his fellow Hebrews.

When a foreign slave exhibits these three signs, he is no longer a foreigner, but a Hebrew. Then the rules change.