Seforno: He was saying to Balak that surely he knew that Hashem was the G-d of Yisrael and that consequently, He would only speak good about them.

Oznayim la'Torah: To Balak's admonishment that if he couldn't curse Yisrael, he should at least not have blessed them, but should have remained silent, he replied that that too was not possible, since he was under strict instructions to say whatever Hashem told him to say - even if it meant blessing Yisrael.