Why did Bil'am add the word "Imachem"?
Rashi (on Avos, 5:9): "Lo ... Imachem, Ela Im Acherim').
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) honor.
Rashbam: Following the words of his emissaries, who reiterating Bil'am's message when, in Pasuk 14, they too, added the word "Imanu". 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 was customary that he said to him (in Pasuk 37) "Am I not able to pay you whatever you ask!"
Rashi writes that Bil'am said "Imachem" to hint that they are below his honor, for he was conceited. He merely conveyed what Hashem told him, "Lo Selech Imahem"!
Moshav Zekenim: Rashi infers that he was conceited from the word "Lesiti". It was unnecessary; he added it due to his conceit.