Quote from The Kin2:
nothing.
Not exactly.
They buy US Treasuries with printed money and hence put an asset on their books. They only pay for the cost of printing.
A debt instrument, especially one that is backed by collateral, is an asset. Any government bond is backed by the power of taxation.
So the Fed can always boost up reserves by going out in the open market and buying US treasuries for pennies on the dollar. Then through the magical power of fractional reserve banking, member banks loans multiply off those reserves.