Mark Cuban, billionaire and owner of the Mavericks, has spent much of the summer tweeting about how terrible Donald Trump is. On Friday, Bloomberg published a series of emails between Cuban and one of their reporters, detailing exactly why Cuban turned against the Republican he once called “probably the best thing to happen to politics in a long, long time.”

In response to Cuban on Friday, Trump said that Cuban “isn’t smart enough to understand what we are doing. He’s really not smart enough in my opinion to really understand what’s going on.”

Cuban has now taken to what appears to be both his and Trump’s favorite medium — Twitter dot com — and thrown down the gauntlet, challenging Trump to a four-hour long interview about his policies. Cuban said that if Trump agrees to let him interview him, Cuban will donate $10 million to the charity of Trump’s choice. Or, if the candidate is that hard up for cash, he can just keep it.

.1) @realDonaldTrump $10mm to the charity of YOUR choice if you let ME interview you for 4 hrs on YOUR policies and their substance. — Mark Cuban (@mcuban) September 16, 2016

2) @realDonaldTrump groundrules are that you cant mention the Clintons or discuss anything other than the details and facts of yr plans and — Mark Cuban (@mcuban) September 16, 2016

3) @realDonaldTrump and no one else is in the room to help. Just me, you and a broadcast crew. Deal ? — Mark Cuban (@mcuban) September 16, 2016

4)@realDonaldTrump I'll add an option.If you need it, I'll write you the check and you can keep the money rather than give it to charity — Mark Cuban (@mcuban) September 16, 2016

5) @realDonaldTrump In the inmortal words of YOU. "What do you have to lose ?" — Mark Cuban (@mcuban) September 16, 2016

Shots fired.