Sifrei on Deuteronomy
לא תסגיר עבד אל אדוניו. מכאן אמרו, המוכר עבדו לעכו"ם [או] לחוץ (לא) [לארץ] - יצא בן חורין.
(Devarim 23:16) "Do not deliver a servant to his master": From here they ruled: If one sells his servant to an idolator or to one outside of Eretz Yisrael, he goes free.
אשר ינצל אליך מעם אדוניו. לרבות גר תושב.
"who shall escape to you from his master": to include a ger toshav ("a sojourning stranger," a gentile who takes it upon himself not to serve idolatry and to heed the seen Noachide laws)
עמך ישב. (ולא) בעיר עצמו.
[If he leaves the gentiles and comes to Eretz Yisrael, do not return him to them.] (Ibid. 17) "With you shall he dwell" — in the city itself.
בקרבך. במקום שפרנסתו יוצאה.
"in your midst" — and not in an outlying district. "in the place that he chooses" — where he can find sustenance.
[באחד שעריך]. בשעריך, ולא בירושלים. כשהוא אומר "באחד שעירך", שלא יהיה גולה מעיר לעיר.
"in your gates" — and not in Jerusalem. "in one of your gates" — and not in exile from city to city.
בטוב לו. מנוה הרע לנוה היפה.
"where it is good for him" — from an inferior location to a superior one.
לא תוננו. זו אונאת דברים.
"you shall not oppress him" — verbal oppression.