But it is not clear the stake would have any value, since Fraud Guarantee does not appear to have any customers. Mr. Giuliani did not make any commercials for the company or otherwise engage in any public marketing for it. Mr. Parnas, who was arrested last month on federal campaign finance charges, has opened talks with congressional impeachment investigators about his role in the Ukraine pressure campaign.

The disclosure that Mr. Gucciardo provided the money that put Mr. Giuliani in business with Mr. Parnas fills in some blanks in a story that has left Mr. Giuliani under federal investigation for possible foreign lobbying violations, and Mr. Trump facing the likelihood of impeachment by the House.

The payment is another example of how Mr. Giuliani, a former federal prosecutor and New York mayor who is facing the prospect of a potentially costly divorce, capitalized on the spike in demand for his services after he signed on as Mr. Trump’s personal lawyer in April 2018.

And it helps explain how Mr. Parnas came to suggest that he and another Soviet-born South Florida businessman, Igor Fruman, had the blessing of a lawyer for the president of the United States as they pursued business and political ventures around the world, including in Ukraine.

In an interview, Mr. Giuliani called the arrangement a “perfectly legitimate payment” for which Mr. Gucciardo received “promissory notes that were convertible into a percentage of the company.”

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Mr. Giuliani said he “verified the stock agreement” and “felt comfortable getting it from somebody as reputable as him,” adding that, after the payments, “I’ve gotten to know Charles very well. He’s a very honorable man, a very good man.”

Mr. Zelin said Mr. Gucciardo did not pay Mr. Giuliani personally, but rather made his investment by paying one of Mr. Giuliani’s consulting firms, Giuliani Partners. Mr. Gucciardo was “simply one of numerous other investors in the company and is nothing more than a passive investor,” Mr. Zelin said.