Banks either use Fair Value accounting or Accrual accounting for a given position. If it’s fair value, then the bond is already marked down on the balance sheet. Selling it or not does not result in a profit or a loss, at least not a significant one. If it’s in accrual, it’s held at par so there is no loss on the balance sheet, but they also can’t sell. They need to hold to maturity, that’s the rule.