I'm sure you didn't mean this, but let's be clear the 6T is not returned to the tax payer that stumped it. Since this isn't generated wealth, just redistributed, it's a cost to someone. Someone's getting 20k, and someone's paying for it. And some people are going to be paying a lot more than 20k, and some people won't be paying in anything.
300M * $20k does equal $6 trillion but it is returned to the tax payer who stumped up the $6 trillion.
The 50M who aren’t contributing tax represent an actual cost of $1 trillion. The remaining $5 trillion is redistributed.
250M people contribute $20k. 300M people receive $16.6k.
300M * $20k - $6 trillion == $0