As WOY said, Castillo's a free agent after this season. If the Marlins want to drop payroll, they just won't try to sign him. He's a free-agent target, not a trade target.
As for Burnett, he might be arbitration-eligible, I'm not sure if he'd qualify as a "super-two" or not. Even then, put yourself in the Marlins' position and ask yourself, if you're going to trade someone as hot and coveted as Burnett, is that deal the best you could do? Probably not, no matter how many second-tier prospects you throw into it.