Trump says the offer is about encouraging transparency. But Letterman didn't exactly sound all in.
"I like to think of the president as being above coercion or simple blackmail here," he said.
"Well, it's not blackmail," Trump replied.
Letterman asked why Mitt Romney hasn't released all of its tax returns, prompting Trump to think of yet another deal.
"I'll tell you, the swap I like?" Trump said. "His tax returns for President Obama's college records and passport records."
