John Gill Commentary 1 Kings 20:9

"Wherefore he said unto the messengers of Ben-hadad, Tell my lord the king, All that thou didst send for to thy servant at the first I will do; but this thing I may not do. And the messengers departed, and brought him word again." — 1 Kings 20:9 (ASV)

Wherefore he sent unto the messengers of Benhadad
Upon the advice the elders had given him, and encouraged thereby, though in a poor sneaking manner after all:

tell my lord the king, all that thou didst send for to thy servant at
the first I will do ;
owning him as his lord, and himself as his servant, and promising to grant his first demand, though so insulting, in the sense he understood him, of paying tribute to him for it:

but this thing I may not do ;
to have not only all put into his hands, but his and his servant's houses to be searched and pillaged, because the elders of his people would not agree; and yet he seems to speak as if he himself would have submitted to it, but was restrained by his council:

and the messengers departed, and brought him word again ;
reported to Benhadad the answer they received from Ahab.