An anonymous donor has promised to give $5 million to a veterans charity of Donald Trump’s choosing if the presumptive Republican presidential nominee releases his tax returns, political operative and Clinton backer David Brock plans to announce Wednesday…

A release previewing a conference call with Brock scheduled for Wednesday says the $5 million challenge “represents an opportunity to fill in some of the unknowns about Trump including how much he has made off of potentially fraudulent ventures and how much he’s actually paid in taxes.

“What we do know about Trump’s taxes shows that he’s gone to great lengths to pay as little as possible,” the release continues. “Meaning that he is avoiding paying his fair share and shifting that burden to other taxpayers.”