Actually the way this works is that the Fed can "buy" as much of this stuff as it wants to by just turning the crank. Doing so diminishes the size of the chair, and also the value of those remaining stashes of notes.
ML/NJ
I know how it works. The problem is that when the Fed buys Treasuries, it is highly inflationary compared to when legitimate buyers like foreign nations purchase them. In short, the taxpayer/citizen gets the shaft through “the hidden tax of inflation.”