They, too, would have a larger capital stock that would produce income that could be used to retire the debt that they inherit from us along with the larger capital stock. The monetary base has its name because it is the base of the financial system. The rest is bank reserves. If the Treasury deposits its surplus at the Federal Reserve banks, the monetary base falls. The reason is that the surplus drains money from the public.
Banks lose deposits.
They pay the Treasury by reducing their reserve accounts at the Federal Reserve. Reducing the base would shrink the financial system, appreciate the dollar, and deflate the economy. The price level would fall. The transitional costs of deflation can be very large, as we learned in the 1930s and as the Japanese continue to learn. Deflation would depress the economy and eliminate the budget surplus.