Wait, there's a real M3 on top of the CMMG he wants to trade for (reconditioned or not)?
If so, this new critical info on the sight and rail makes a difference since this doubles the value in the trade. You have ~$1,000 - $750 to trade.
Even if you're getting both pistols (BTW, you do mean the Argentine FM Hi-Power?) then the trade isn't that good considering you could take the M3 off and sell it and then sell the CMMG for more than the used value of the two pistols and your wife could buy a .22lr rifle she actually liked as well as a .22lr pistol.