1934 $500, Valuation?

Unless you have the cash already in your pocket, I see the dilemma, since the bank is not going to lend you that money to buy one of the notes. Best workaround I can see is if you have an account there, then you could withdraw the $500 from your account and exchange it for the note. That way the bank is not liable for or in possession of the dubious note at any point in time. The only issue with this would be the possible appearance of mixing personal transactions with bank transactions, but I hear the policy on these things changes from bank to bank.