The balance sheet can be unwound simply by waiting for the bonds to expire, then the money goes poof out of existence the same way it poofed into existence.
How would that work? The money's been created and has been put into the economy. Balances on bank accounts have increased. You can't just take it back just like that.
When the bond matures it must be paid back, and the money that was created when the Fed purchased it is now destroyed as the Fed receives the bond principal.
If the bond is not paid back and the debtor defaults, well... this can't happen with US Treasuries because the fed is an arm of the government and will always roll over a bond. But if it happens with the corporate bonds that the Fed is buying, then the US Treasury is on the hook for the losses because by law the Fed cannot be impaired.
How would that work? The money's been created and has been put into the economy. Balances on bank accounts have increased. You can't just take it back just like that.