It was all for a roast on Comedy Central -- hosted by Seth MacFarlane -- The Donald volunteered to be the human punching bag ... and TMZ has learned, he walked away $2 million richer.
According to sources, Trump was completely against taking part in the show -- but when he found out how much he'd get paid ... he stepped right up.
Thankfully, we're told Trump plans to donate his ENTIRE paycheck to charity -- but so far, no word on which good cause stands to profit off his bruised and battered ego.