Hi, that looks very serious, as I think you've already determined. Two things - either the shipper sent it to you in that condition (it does look infected to me) or the fish rubbed its jaw during shipping (I've seen this happen before, especially with Rhinopias scorpionfish). What happens is that normally sedentary, bottom dwelling fish get put in a shipping bag and they freak out and just try to swim the entire time - damaging their lower jaw.
So, after calling the dealer, what to do? There isn't any mechanical fix you can do - I've had veterinarians try to use cyanoacrylate glue to fix things like this, but usually without success. Luckily, anglers can go a long time without food. I would hold it in a dimly lit treatment tank and dose it with a broad spectrum, gram negative antibiotic and see if it can heal on its own. You could use Furan-2 or Neoplex.
Jay