The co-author of President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE's 1987 best-selling book, "The Art of the Deal," says special counsel Robert Mueller's investigation will find that the president is worth a lot less than he claims.

One thing Mueller will likely find: Trump is worth VASTLY less than he says. More debt, less cash. He's desperate not to have that exposed. — Tony Schwartz (@tonyschwartz) September 1, 2017

Earlier Friday, Tony Schwartz also tweeted that there is "no way" the president will follow through on his pledge to donate $1 million to the victims of Hurricane Harvey.

"He only promises to give. Never actually does," said Schwartz, a frequent Trump critic.

On Thursday, it was reported that Mueller had begun working with agents from the IRS who specialize in prosecuting financial crimes such as tax evasion and money laundering.

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Mueller has also reportedly been working with New York Attorney General Eric Schneiderman (D) to investigate former Trump campaign manager Paul Manafort’s potential ties to Russia. A Russian lobbyist who attended a meeting between Trump campaign officials and a Russian lawyer have testified before Mueller’s grand jury as part of the investigation.

Dan Rather said Thursday that Trump is "seized with fear" over the investigation.

"Donald Trump is afraid," Rather said on MSNBC. "He's trying to exude power and strength. He's afraid of something that Mueller and the prosecutors are going to find out."

This isn’t the first time that Schwartz has been critical of Trump. The "Art of the Deal" co-author has spoken out against the president since he took office, predicting that Trump would resign by the end of the year.