AFAIK there is no 'back door route'. I would think that the CPW salesman would assume that you knew the terms & conditions of your contract with T Mobile. It is standard practice throughout the industry that if you sign up to a fixed term contract, you pay for that term whether you want to end the contract early or not. Frankly, you should have done your own research and been aware of the terms of your contract. It is not a penalty fee, it is the contractual figure remaining for the full term of the contract you signed.
In your position, I think it would be wise to return the iphone and see out the end of your contract with T Mobile and then go for the phone of your choice in January.
But that's just my opinion, ultimately the choice is yours.
I don't think this is what you would want to hear but maybe someone on the forum will be able to advise you further.
Good luck