Run away. There are too many casinos that won't ask you to do that, which looks like a scam. In a bank it's reasonable, but not in a casino.
The problem is that I have money to withdraw
What is the name of the casino? I asked for us to know if it is legit or not. If the casino is asking you for money again without able to withdraw, it can likely be a scam casinos.
If you have answered that question and the casino is legit, all you need is proof of income or something related to it statement if account.