Who offered to return cities?
Rashi: Ben Hadad offered to return the cities that his father took from Amri, Achav's father.
What are "Chutzos"?
Rashi (Yirmeyah 9:20), Radak: They are markets. You will get taxes from what is bought and sold there.
Why did he offer to him markets in Damesek?
Rashi: It is my capitol. You will rule there, just like my father did in Shomron, Bal Korchacha (against your will). Ben Hadad said all this amidst fear.
Who said "va'Ani ba'Bris Ashalcheka"?
Rashi: Achav said so to Ben Hadad.
Why is there a Patach under the Beis in ba'Bris, which implies that it was mentioned above?
Rashi: It is this Bris (Radak - on condition that you fulfill what you said).
Malbim explains that he made a Bris of brotherhood. Perhaps ba'Bris alludes to the brotherhood mentioned above ("Achi Hu... Achicha" - PF.)
Was it proper to send him away?
Radak (35): No. Mercy on Resha'im is cruelty, for it is known that in the end, they will make war. Similarly, Sha'ul had mercy on Agag, and Hashem was angry (Koheles Rabah 7:16).
Malbim: This was foolish. Achav did not even demand the cities that Ben Hadad offered to return 1 .
Malbim: This was a pitfall for Achav later (Perek 22). "The mercy of Resha'im is cruel" (Mishlei 12:10)!