Sforno on Genesis 16:6/ספורנו על בראשית ט״ז:ו׳

1שפחתך בידך, for she has not been given her freedom by you who are her mistress.

2 ותענה שרי, Sarai's objective in maltreating Hagar was to remind her of the fact that she remained her slave. She was not to insult her mistress any more. She meant to make clear that any gentile who insults Israelites will experience similar harsh treatment. Compare Isaiah 60,14 והשתחוו על כפות רגליך כל מנאציך, "all those who reviled you shall prostrate themselves at your (Israel) feet."