Rashi on Numbers 22:34/רש"י על במדבר כ״ב:ל״ד

1כי לא ידעתי FOR I KNEW NOT — This statement, too, (cf. v. 29), was a disgrace for him, for in spite of himself he had to admit it — because he used to boast that he knew the will of of the Most High (24:16) and now his own mouth bore testimony: “I knew not” (Midrash Tanchuma, Balak 10).

2אם רע בעיניך אשובה לי IF IT BE EVIL IN THINE EYES, I WILL RETURN — To set himself in opposition to the Omnipresent was the purpose of this reply. He said, in effect, “He Himself commanded me to go, and you, angel, annul His words. He is accustomed to this — that He says something and an angel recalls it. He said to Abraham, (Genesis 22:2) Take now thy son, etc.’ and by an angel annulled His words. I, too: "If it be evil in thine eyes, then I must return” (Midrash Tanchuma, Balak 10).