Rashi on Genesis 28:15/רש"י על בראשית כ״ח:ט״ו

1אנכי עמך I AM WITH THEE — God promised him this because he was in terror of Esau and Laban.

2עד אשר אם עשיתי UNTIL I HAVE DONE — The word אם is here used in the sense of כי (cf. Gittin 90a).

3דברתי לך means I have spoken in thy interest and concerning thee: whatever I promised to Abraham regarding his seed it was in reference to you that I promised it and not in reference to Esau, for I did not say to him “Isaac shall be called thy seed” (which would have signified that all Isaac’s descendants would be regarded, also, as Abraham’s) but I said (21:12) ביצחק “In Isaac” and not all the issue of Isaac”. In the same way wherever לי or לך or לו or להם are used after a verbal form of דבר, they are used in the sense of “concerning”. This verse proves that this is so, since it cannot mean “I have spoken to thee” as He had never spoken to Jacob before this occasion.