Like every other big-time real estate and construction tycoon in NYC, Trump probably had to pay off Mob-dominated unions and contractors. In "Underboss," Sammy Da Bull uses Trump's name in an example of how he used his union connections to extort real estate and construction moguls. He didn't exactly say he really did extort Trump, but Trump probably paid off the Mob one way or another.
Two other points BTW:
1. During the 1984 Presidential campaign, the media uncovered that real estate agent and developer John Zaccaro, husband of Democratic VP candidate Geraldine Ferraro, owned NYC buildings that housed Mob-dominated businesses. One pal noted that it was a measure of Democratic Presidential nominee Walter Mondale's naivete that he didn't figure that his VP candidate's husband would have skeletons in his closet simply by being in the real estate business in NYC. Had nothing to do with being Italian, though the bigots and sexists had a field-day with the revelation.
2. Trump, the all-time publicity hound and egotist, probably enjoyed being mentioned in Da Bull's book. When former Soviet Premier Gorbachev visited NYC in the late '80's, Spy Magazine (a satire magazine) scammed Trump by hiring an actor who looked like Gorby to pose outside Trump's office building. The magazine called Trump and told him that the "Soviet Premier" was a great admirer of his and was waiting outside to shake his hand. Trump appeared instantly, posing for still and TV cameras with "Gorbachev." Spy immediately told the media about the scam, and all of them ran satirical pieces on it.