If it's an investment ask for the contract of sale or other relevant documentation. An investment of this size, if it's legitimate, should have some kind of documentation. I'm not sure you can deny someone their money, but I'll defer to others on this matter. However, you can absolutely tell the customer that you will need a few days to prepare that much cash. Something like this our legal counsel would require the customer to execute a release advising them that the Bank warned them against taking that much cash due to various risks involved.
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"100 victories in 100 battles isnt the most skillful. Subduing the other's military w/o battle is the most skillful." Sun-Tzu