He had a valid case against DeNiro's character
(spoilers)
So you sell your soul to the devil to become a famous singer.
Pretty soon after you get famous you get drafted, injured in war, and lose everything including your fame. And then the devil dares to show up to get that soul?
I understand the devil is a son of a bitch and will cheat his way out to get your soul, but there is no way what happens here after the sale respects and satisfies the conditions of the contract.
I'm sorry for the devil but losing your fame and identity will make his end of the deal invalid, unless he could be restored as that famous singer again.