Albert Barnes Commentary 1 Kings 20:34

"And [Ben-hadad] said unto him, The cities which my father took from thy father I will restore; and thou shalt make streets for thee in Damascus, as my father made in Samaria. And I, [said Ahab], will let thee go with this covenant. So he made a covenant with him, and let him go." — 1 Kings 20:34 (ASV)

Ben-hadad, confident his life would be spared, proposed terms of peace in exchange for his freedom. He would restore to Ahab the Israelite cities his father had taken from Omri, among which Ramoth-gilead was probably the most important (1 Kings 22:3). Furthermore, he would allow Ahab the privilege of establishing for himself “streets,” or more accurately, trade districts, in Damascus—a privilege his own father had possessed in Samaria. This arrangement was likely sought for commercial advantages more than any other reason.

So he made a covenant with him. Without inquiring of the Lord, Ahab immediately agreed to the proposed terms. He allowed the Syrian monarch to depart without even taking any security to ensure the terms would be honored. Politically, this was an act of culpable carelessness and imprudence. Ben-hadad did not consider himself bound by the terms of a covenant made while he was a prisoner, as his later actions show (1 Kings 22:3). Ahab’s conduct was even more unjustifiable for someone who held his crown under a theocracy. “Inquiry at the word of the Lord” was still possible in Israel (1 Kings 22:5, 8), and it would seem that ordinary gratitude alone should have suggested this course of action.