There was a scene in The Life and Times where a Bad Dude dropped a younger Scrooge onto his money pile from a very very great height, and Bad Dude was expecting Scrooge to go THUD-SPLAT and die, and Scrooge was expecting to go THUD-SPLAT and die, and it's a toss up as to who was more surprised when instead of THUD-SPLAT, Scrooge went SPLASH. And that is how Scrooge discovered his mutant money-swimming powers. :-)
And as stated, yeah, Scrooge does know exactly how much money he's got, but the IRS probably has massive headaches trying to deal with it anyway, because Scrooge is so gosh-darned rich, they have to invent new numbers to describe the amount. How much is an "impossibajillion" dollars, and how much does he owe on that?