I think it's easy to payback the $31 trillion dollars. Have the president direct the Secretary of the Treasury to print up a $31 trillion dollar note which is then handed to the Federal Reserve as completely payment. The tricky question is what to do in the next few seconds/minutes. Personally, I would swallow the entire pill and have the Treasury Department immediately introduce a new asset-backed currency which has a maximum convesion payout from dollars to the new currency for each individual adult citizen of $50,000 (or $100,000 or $500,000 - some reasonable number) and at least for some time period (maybe forever) not be convertible to any foreign currency. That will get rid of all the billionaires and many multi-millionaires (no one needs billionaires anyway). That's at least one way to handle it. (edit - tillion to trillion)