Its a sketchy deal. Bill is using his left hand so the deal isn't an official deal (right hand) meaning he could change the deal as he seems fit and the summoner gets screwed. Was inspired on how quickly Gideon made the deal with Bill, leaving Bill to decide what he would get in return since he didn't state what he wanted so he could change/add the deal.
Bill: You know what kid. You've convinced me. I'm sold! I'll help you with this and in return you could help me with something I've been working on. We'll work out the details later.
that part made me laugh so hard
It took me two hours to draw this. Who know drawing a triangle took so long. I'm going to make this into a shirt and make the background birch trees just how Bill was summoned. Bill Cipher is my favorite character.