Commentary for Numbers 22:34

כי 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).

ויאמר בלעם…חטאתי, Bileam said…"I have sinned, etc." The word "I have sinned" referred to the angel having hinted that he had sinned by his having set out on the way. Regarding the angel's question of why he had beaten his ass, Bileam said: "I did not know." The word ועתה means that Bileam had now become a penitent. He no longer wanted to curse the Jewish people Concerrning the fact that he had set out on the way, Bileam offered that if this displeased the angel he was willing to turn back. The word לי meant that he now saw that it would be beneficial for him to go back home.

ויאמר בלעם אל מלאך ה' חטאתי, ”Bileam said to the angel of the Lord: ‘I sinned;’” our sages in Tanchuma Balak 10 state that this verse is proof that if someone confesses his guilt by saying “I have sinned,” the angel no longer has permission to touch him.