Bad idea, replace all the pistons and rings, not just one.
If the engine is apart and the bottom end already torn down, replacing only one piston is a waste of time.
If you are really on a budget and only want to rebuild to standard specs, you could price up factory pistons and rings from Nissan.
That said, if you bought the car 4 weeks ago and you haven't done anything stupid like crank the boost up, I would demand that the dealer wear the entire bill.