Mr. Giuliani’s speeches and travel that year and in the decade since were often related to business deals he was pursuing, or even standing agreements he had reached, like one with backers of the dissident Iranian political party known as the Mujahedeen Khalq, which until 2012 was considered a foreign terrorist organization by the State Department.

They offer a road map of sorts to the kinds of potential conflict of interest questions that are already emerging as Mr. Giuliani’s name is floated as a possible Trump cabinet member, particularly given that Mr. Trump repeatedly mocked Hillary Clinton, his Democratic opponent, for the many speeches she and her husband gave after they left public office.

“When she left, she made $21.6 million giving speeches to Wall Street banks and other special interests in less than two years — secret speeches that she does not want to reveal to the public,” Mr. Trump said in June. “Together, she and Bill made $153 million giving speeches to lobbyists, C.E.O.s and foreign governments in the years since 2001. They totally own her, and that will never change.”

In particular, Mr. Trump criticized Mrs. Clinton’s speeches to Wall Street banks. But Mr. Giuliani, in a one-year period that ended in January 2007, earned $750,000 in speaking fees — before the 20 percent cut taken by the speakers’ agency — for eight speeches he gave to Wall Street banks and other major financial institutions, including JPMorgan Chase, Merrill Lynch and Lehman Brothers, before its collapse. As with Mrs. Clinton, there is little public record of what Mr. Giuliani said during these events.

Mr. Giuliani had rules about keeping his remarks private, as a contract “addendum” to which organizations that invited him to speak had to agree makes clear. Mr. Giuliani stipulated that his remarks could not be recorded, nor could “general press or other media coverage” of the remarks be allowed without his explicit permission. He also had some elaborate demands, including that if he traveled by private plane, it be a Gulfstream IV or bigger, a plane that costs about $40,000 for a one-day trip within the United States.