How to unlock a sprint Iphone 5s for which I paid full price?

On a trip to the US, planed especifaclly to be after the release of the new Iphone, I bought a sprint Iphone 5s for full unlocked price ($650) to be used overseas from a apple store in dadeland, Fl. I discussed my intention of use overseas with apple staff, who told me it should work but if I need to use it in the US it will only work with sprint. I got to my country, El Salvador, and to my suprise it gives an invalid SIM message. After contacting Apple customer service it appears apple can't unlock the phone remotly, Sprint has to do it. I called Sprint customer service,who after a 2 hour wait, tell me they can't unlock the phone until I have at least 3 billed months with Sprint. This was never said by apple staff at the store. After calling apple customer service again they tell me I can take it back to the store for an exchange, which would be great if I wasnt in a different country making it inpossible.

I am a long apple customer having purchase from them an endless amount of products and their customer service has been real friendly and trying to help with my problem, even calling me to give me update. But at the end of the day I just want to use the phone which I purchased so I need a solution to my problem.

How can I unlock a phone for which I paid full unlocked money to be used in my country, without hacking it or jail breaking it?