Was the entire story false?
Malbim: No. He really was in the war. The "Ish" was "Hashem Ish Milchamah" (Shemos 15:3). He was part of Klal Yisrael, who needed to guard Ben Hadad. For not guarding him, all were punished - Achav would be killed, the Navi was hit, and Yisrael lost money (Aram plundered them) - "Kesef Tishkol."
What is the meaning of "Ish Sar"?
Rashi: He veered from the road to come to me.
Whom was he told to guard?
Rashi: It was a captive.
What is the meaning of "Im Hipaked Yipaked"?
Rashi: If he will flee. This is like "v'Lo Nifkad Mimenu Ish" (Bamidbar 31:49), an expression of omission.
What is the meaning of "Oh Kikar Kesef Tishkol"?
Rashi (40): [If he flees you will die], or pay a Kikar of silver 1 .
Ralbag: Or, [if you guard him properly], you will receive a Kikar of silver.
Tanchuma (Ki Sisa 11): Moshe learned from here that the redemption of a soul is a Kikar (3,000 Shekalim), so Hashem needed to tell him that everyone will give only a half-Shekel.
What was the need for this story? Even without it, the Navi could say, you are liable for letting him go!
Radak (38): Hashem wanted that Achav will give the ruling for himself.
