Why did Bil'am add the word "Imachem"?
Rashi: This teaches us that Bil'am was conceited and that he wanted to hide the fact that he was under Hashem's jurisdiction.
Implying that Hashem had refused his request the first time in defense of his (Bil'am's) Kavod.
Following the words of his emissaries, who reiterating Bil'am's message when, in Pasuk 14, they too, added the word "Imanu" (Rashbam). See Ramban's objections to this explanation.
Ramban: In whose name he proudly spoke.
Ramban: And it was because Balak suspected him of expecting a higher wage than usual that he said to him (in Pasuk 37) "Am I not able to pay you whatever you ask!"