So yes, uncle sam can steal the money in your mattress without having access.
Or the Fed could release a newDollar and only accept a certain amount of oldDollars. Russia (I think) has done that once or twice.
2/3 of oldDollar cash is stored outside the US by people who mostly can't enter the USA without visas etc. And those will be pissed royally.
It would be not wise choice to piss off more people than your own population, however not never done by the USA.
So yes, uncle sam can steal the money in your mattress without having access.
Or the Fed could release a newDollar and only accept a certain amount of oldDollars. Russia (I think) has done that once or twice.