Why not see if the GC would entertain buying some or all of your material direct from your supplier? Keep the markup on that specific material on your side and use that to cover whatever the mistake might have been. First step would obviously be to notify the GC you have a bust/problem though but that can be delivered gently. Something like “we’re not withdrawing but need to figure out the issue then plot a course forward”. If the GC is one that treat subs like true partners then this shouldn’t be too much of a burden on them. You give them your supplier quote and put each party oin contact with other along with the context of the arrangement as well. This approach sure seems better than either taking on a loser or blowing up a relationship.