How did he get $250m for bail to begin with?
The bail amount is the penalty for violation; while in the state systems it tends to have a simple mathematical relationship to the price the defendant pays for a bond (often regulated to around, or sometimes fixed at exactly, 10%), in the federal system it is more fluid.
If he doesn't have/can't borrow the full bail amount, I see no reason for him to be out on bail. Normal people don't get this kind of privilege.
A bank is private business they can choose how to go about their risk and reject anyone for any reason the criminal justice system needs to be fair to all the participants in it .
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[1] yes , there are plenty of people in jail only because they are unable to post bail or some other fees but that is not the same debtors prison