The debt is in currency. The coin is not offered as a token of currency, but as a piece of silver. It's not being offered as a bribe.

"I'll give you this mystery box in exchange for discharge of all past debts I owed you". If you accept the mystery box, you cannot claim the debts. If the bank knowingly accepts and deposits the "silver coin" under these conditions, the debt would theoretically be discharged.

Of course, making an offer in this way is explicitly prohibited under postal regulations: The recipient is under no obligation to abide by the terms of such an offer, and is free to keep the coin without it constituting acceptance. The bank is under no obligation to return the coin.