Rashi on Genesis 31:52/רש"י על בראשית ל״א:נ״ב

1אם אני THAT I WILL NOT — Here the word אם is used in the sense of אשר “that”, as (Genesis 24:33) “until that (אם) I have spoken my words”.

2לרעה FOR EVIL — For evil you may not pass, but for trading purposes (as e. g., with a caravan) you may pass (Genesis Rabbah 74:16).