I think we’re talking about bank reserves, which is a fraction (in the order of 1%) of the total amount of money held in the customers’ transaction accounts. Reserves are convertible into cash. Not that any bank would suddenly want to do that, unless there’s a bank run, in which case it’s the customers who want the entirety of their accounts (100x the reserves) converted into cash, which is impossible not because the fed refuses to convert the money, but because the bank doesn’t have enough reserves.