Metal sensitivities get worse as they are constantly exposed to you. A tiny amount can still cause you to be reactive and we're talking 24/7 contact with your oral and GI tract, I would not risk it. Especially if risk was HIGHLY reactive, not just reactive. Even the tiniest amount of nickel in jewelry can cause people to break out.
I've learned to get it done RIGHT the first time, cause if you're going to have to go back and change it later (or worse, spend more money figuring out what it is thats causing problems and THEN replace it) youre paying a lot more.
Ask the dentist to refund the product and you'll pay the restocking fee. I dont see why you'd have to pay retail. Or go to a different dentist if he's being stubborn.