The way they remove money from the money supply is to sell the assets from their balance sheet. If they only get half as much for them as they paid, then they still have half of the money on their balance sheet and in the banking system. They can not pull out all of the money they pumped in. Your brilliant exit strategy is a failure.
Then they sell twice as many bonds.
The 'issue' you are bringing up is only a problem if the Fed intends to remove ALL the money from the money supply, and run out of assets to do so. Something tells me that abolishing the Federal Reserve Note is not on Bernanke's to-do list.