An investor who employed and was close to Jeffrey Epstein is scheduled to appear before members of Congress investigating the deceased sexual abuser.

Billionaire Leon Black paid Epstein extraordinary sums that his spokespeople have said was for tax advice and estate planning. Earlier this month, Democratic Sen. Ron Wyden of Oregon wrote to the House Oversight Committee — which is set to interview Black on Friday— demanding they ask Black about some $170 million in payments to Epstein between 2012 and 2017.

"To date, I do not believe Black has provided a credible explanation as to why he paid Epstein amounts that vastly exceeded those paid to other professional advisors involved in his tax and estate planning," the Oregon senator wrote.

Wyden also wrote to Black in March to demand answers to questions arising from revelations from the Epstein files about his "significant personal and financial entanglements with Epstein."

On Thursday, asked whether Black had responded to his letter, Wyden said, "He stonewalled repeatedly. We just haven't gotten the answers that are responsive."

An attorney for Black, the co-founder of Apollo Global Management, wrote in an April letter to Wyden that documents released this year under the Epstein Files Transparency Act "do not contain any credible evidence that Mr. Black was aware of, or involved with, Mr. Epstein's then-ongoing criminal activities." 

Another of his attorneys told CBS News in December that an internal investigation at Apollo "concluded that Mr. Black paid Epstein for estate planning and tax advice, no more, no less."

Emails released by the committee last year document Epstein was involved in Black's personal matters while providing wealth management advice. Black had a six-year affair with a former Russian model, which ended in acrimony and allegations of abuse, according to court records.

As Black prepared a nondisclosure agreement in 2015 to secure the model's silence, Epstein offered him advice, including suggesting in an email that Black hire former law enforcement officers to approach her.

"Choose method of message delivery, my choice. - two highly respected former ---- fill in the blank, immigration, scotland yard. sfo. . who may knock on her door and present the terms," Epstein wrote. His writing frequently eschewed basic grammar and punctuation. 

The nondisclosure agreement broke down four years later, leading to a series of lawsuits and countersuits. Court filings show Black discussed the agreement with just two people: Epstein and a private investigator. 

The internal investigation at Apollo, which was conducted by the law firm Dechert LLP, concluded that while Black had "confided" in Epstein about personal matters, he paid Epstein for wealth management. The investigators wrote "that Black genuinely believed that Epstein was extremely smart, capable, and saved him substantial amounts of money."

Black is among several billionaires to testify before the House Oversight Committee, which also interviewed Microsoft founder Bill Gates, Commerce Secretary Howard Lutnick and businessmen Les Wexner. 

Others who are expected to be interviewed in the next month include former Goldman Sachs executive and Obama White House lawyer Kathy Ruemmler, and Harvard law professor Alan Dershowitz and former JPMorgan Chase executive Jes Staley.