Bummer about the mismatch receiver cover.
$319 isn't bad, but like AH says, you can probably do better. Toss out a $250 number at them, let them know it's simply never going to appreciate like a collectors piece because of the mismatch, and see what they say. Maybe you & the shop can meet in the middle at $285 OTD or something. Maybe you can negotiate for them to toss in 100 rounds of ammo or a 20 round Chinese star magazine or something else they have laying around.
Pawn shops will most certainly negotiate, they probably acquired that gun for under $200 and it's not doing them any good sitting there unsold, especially during the slow weeks at the beginning of the year when people realize they spent too much during Christmas and they tighten their belts due to new years resolutions.