If you have already assembled the breastplate and dragon heart, then you can go back to grischa and trade to get the gem back! It will be seperate, so you can trade to get the original dragon armour back (not that there is any point) but anyway, just trade with grischa to get the gem back, show it to Alrik, then go back to Grischa and trade it back to get most of your gold back (or if you are already rich you may want to keep the gem)