I'm not an expert by any means, but I think more details are required. What Red Dot, barrel length, barrel lining, barrel twist rate, and anything else with the AR along with an approximate round count for both.
I would try to talk him down on the cash portion then. Before the December buying craze I built a RRA lower with all PSA parts and magpul furniture from PSA for about $650. I don't know what the same rifle would cost now.
Looks like a standard M4 , CL 16" with a MI hand guard , other than that its nothing special . I bought one of these with the M4 hand guard for $626 to my door in dec of 2011 . I still have it and its the one I turn to most even with all the fun toys I have . The red dot is nothing special to jump up and down about so its not much of a added bonus . Depends on what you want . Do you use the glock alot ? Do you have another pistol ? Can you get ammo for the AR at a reasonable price ? Lots to think about