Mark Cuban says Donald Trump's $5 million challenge to Barack Obama was "one of the DUMBEST things ever" ... and now he's responding with a $1 MILLION challenge of his own.
Cuban appeared on Fox4 in Dallas last night ... and made the offer to Trump:
"Donald, you shave your head, $1 million to any charity you want," Cuban said.
Cuban says he's willing to up the offer to $2 million ... if it'll help seal the deal.
So far, Trump has yet to respond ... but let's get serious -- ain't gonna happen.