I have a nickel allergy too. Mostly try to avoid it,
BUT,
in the case of pant buttons, buckles or antique costume jewelry I just have to wear for some reason, I've had success coating the back and sides of the item with clear nail polish, letting it dry, putting on one or two more coats, let dry between each coat.
I've done this to the backs of metal buttons that touch the skin and the inside of costume rings.
It is only temporary. Must re-apply the polish after washing item and when it wears off.