I've never had a company ask for proof of income and I wouldn't work at one that did. That's an insane requirement that should have nothing to do with whether or not I'm fit for the job and they're willing to pay what I ask for someone to do the job.

I would never work for a company that went to that level. In 10 years, I havent seen a company pull that. If you suspect that might be part of their hiring process. Just say "I need to make X" and evade all talk of current salary.

How are they going to verify your current salary? Do companies really go out of their way to do that?