They have choices. They have complete flexibility. They are under no pressure to do anything that isn't explicitly their goals. They want to control inflation and increase employment. If the economy starts to heat up and inflation starts to rise above their 2% target they will then have the option of selling bonds or raising the federal funds rate, the latter of which is more likely.
I'm not really sure what you're trying to get at with the last comment. He doesn't know the exact number off the top of his head. The exact amount remitted last year to Treasury is $88.4 billion.
