The thought of the danger of explosion of the Curese Worm and the risk of damage to his Life Ability is why his face is feeling the phantom pain. He is sure that he will grimace and feel heartache if the Corpse Worm explodes, so he is already grimacing now and grimacing intensely, which is causing his face to actually feel pain.
Unfortunately, he can't back down because he doesn't want to offend Skarg and because he needs the money. He needs the money to start preparing for his advancement to rank 5 or at least prepare a way out for himself if he fails to advance to rank 5.
He doesn't actually want to advance to rank 5. He is satisfied with what he has achieved in life and will be content remaining a rank 4 cultivator all of his life.
