Well congress lets the Federal Reserve control the value of the dollar and that value isn't simply decreed.

The value of the currency depends, in large part, upon how much money there is in the money supply.

So yes the Federal Reserve could take action to quickly decrease the amount of US $ in the system. This is called Deflation and has very specific and severe consequences.

I think controlled deflation would be a good thing for this bloated economy but it couldn't pay back the national debt the way you imagine.

Governments have done the exact opposite before. They say, "Oh we owe $25,000,000.00? I'll just print it up and hand it over. Debt re-paid."

But that has severe consequences too.