The problem, as others have pointed out, is that you are setting up a PVP situation which is generally something to be avoided. One of the advantages of D&D for many people is that it's a cooperative game. Take that away and (in my experience) it tends to spiral out of control.
Personally, I have a hard-and-fast rule of no Evil PCs and this action is pushing him towards that evil alignment and he would have to do something to absolve his guilt. However, I always establish that at the start of the game and I wouldn't just turn a player's PC into an NPC without warning or at least giving them a chance to seek absolution.
Having said that making a deal with a devil rarely works out in favor of the person making the deal. A few options of "blessings" off the top of my head.
Blessed with Luck: The PC gets advantage on skill checks, attacks and so on every once in a while. When this happens, everyone else in the party has disadvantage until the start of the PC's next turn. Gradually increase this "luck" ability so that the PC is incredibly lucky but everyone else around him has horrible luck.
Great Wealth: Similar to Blessed with Luck, the PC always has gold, while gold disappears from PCs and NPCs alike.
Advisor: Someone else mentioned this, but give the PC an invisible imp that gives them advice. The advice will technically be correct, but will frequently be tactics that hurt the most innocent people or endanger other members of the party. If the PC takes the advice and acts on it, they slowly come under the control of the imp more and more until they are dominated.
Black Blade: give the PC a magical weapon of their choice. The weapon absorbs the souls of the people it slays. Absorb enough souls and you get a special ability/attack. Using souls this way corrupts the soul user, slowly turning them into a fiend.
So basically, curses that are also advantageous at least in the short term.
In all cases, getting rid of the "blessing" requires some kind of atonement or minor quest.