Copyright Help?

Hmmm well it depends on your contract or written agreement with each other. Of course, no matter what, he still has to credit you for your work. If not, that is copyright infringement unless you specify that you don't need to be credited. For payment, you could receive money initially after drawing for him for that app, it's up to the terms of the contract if you wish a percentage of money from the sales of that app or not, idk up to you xD;

although, you cannot use those designs you created for the app anymore and claim it as your own. It belongs to that friend of yours now. That's how I understood copyright in terms of selling concept designs to game creators.