When William expressed his acceptance to such a crazy deal, Alfonso didn't hesitate to get out the papers that lacked William's signature on them. He pointed to where William should sign, and explained in rushed words what William shall do. And then everything about this cursed place would belong to William.
"You came quite prepared," even if a kid was sitting in front of Alfonso right now, even if an idiot sat there, they'd suspect something was wrong from Alfonso's rash actions.
"It's just to save your time and mine, hahahaha! You are quite busy indeed, right?" Alfonso tried to find an excuse to give to William to cover up his clearly suspicious actions.
"It's alright," William took the papers, the brush and the pot of ink, and left his name there, "it's mine now, right?"