i would not suggest using the older FID,DL. even though its a face to face private sale.
if you were trying to make a store purchase,they will only let you see and handle the firearm. they will not sell it to you, and the store has to make a phone call too. something about after the attacks on 9/11 made it illegal to use a FID/DL combo without matching info.
you could pay him for the rifle,just be sure to check it over which i have a feeling you did before saying you would buy it. just explain to him that you can't take possesion of it until your papers are straight. be sure that when you get the paperwork straightend out, and pick it up its in the same condition or better than when you paid for it. if you pay for it as long as you don't take possesion of it, you should be clear.