Apple should really just have a single gift card option. An Amazon gift card lets you buy anything on the store. All gift cards are bought with money so it shouldn't matter how it's reclaimed. Understandably, Apple like to hand out prizes limited to the iTunes Store but they can have special Apple redeem cards for that - for anyone else, that restriction shouldn't be needed.
As to a solution, I'd say your only option would be to try and get the money refunded from Apple or try selling the cards if you can for a lower price but I doubt anyone will want $500 worth unless they get a crazy deal for it and even then there's so much spam with gift cards that you'll be hard pressed to get anyone to agree to it.