John Gill Commentary Numbers 22:38

"And Balaam said unto Balak, Lo, I am come unto thee: have I now any power at all to speak anything? the word that God putteth in my mouth, that shall I speak." — Numbers 22:38 (ASV)

And Balaam said unto Balak, lo, I am come unto you
And therefore let nothing more be said of what is past: and as for future things,

have I now any power at all to say anything ?
that would be agreeable to the king, and answer his purpose in sending for him, namely, to curse the people of Israel; he suggests that he had not, he was under the powerful restraint of God; he could not say what he himself was inclined to say, and what the king would have him say, and he expected he should; he could not say as some in (Psalms 12:4) : the word

that God putteth in my mouth, that shall I speak ;
whether agreeable to Balak's will and design or not, and whether for or against Israel: however, it being expressed in such indefinite terms might leave room for Balak to hope it might answer his expectations and wishes.