I'm not going to go into a huge detailed explanation with examples, but what I'm saying is Oda doesn't write in a way that the 400,000,000 bounty is false. There would have been some sort of clue, subtle or not, that the Fake luffy was lying, or had his information mixed up. Either in the way he said it or some other circumstance. He doesn't just insert random information like that and just have lying characters spread false information like that unless it's specifically serving a purpose. Like any other writer does. Once you've taken some writing classes, it makes much more sense.
I'm willing to bet Luffy's new bounty is 400,000,000.