OK.

So there's a lot of backstory to this document.

Trump was associated with a Labor Racketeer named Dan Sullivan.

Sullivan also an FBI informant. This didn't make Sullivan a good guy. This is was more like an insurance policy a lot of guys who were full crooked or just slight bent were FBI informants.

Trump worked with Sullivan even before he went into NJ. And later Trump went into business with Sullivan. They were partners in a drywall company

This was a big problem for Trump. It's 1981 and he is trying to open a casino and he needs a casino license. The problem is the New Jersey Division of Gaming Enforcement won't give licenses to guys with crooked business partners or even slightly bent guys.

To solve this Trump could just cut Sullivan loose. But he doesn't want to do that because Trump is trying to buy/lease parcels of land to put together for his casino. Sullivan owned that land with Kenneth Shapiro, a gangster who was like the banker of the Philly mob. And Trump leased the land to build his casino from them.

And that's not even the craziest Trump working with Atlantic City mobsters story.

There was one piece of land Trump wanted for another parking lot.

On this land was unsuccessful night club.

Unsuccessful because it never got a liquor license.

Why? Because it was owned by the Philly Mob. And not just fully crooked guys either. Straight up killers.

Salvatore Testa was one of the owners. Testa was a hitman who became of a fully made member of the Philly Mob at the age of 24 because he made his bones as a hitman and because his dad was the Boss until he was killed in the most famous bombing in history of the US mob. Famous because another Boss, Bruce Springsteen wrote about the Bombing in his song Atlantic City.

I particularly like Levon Helm and The Band's version https://www.youtube.com/watch?v=QFxXTO1t5yo

Back to the purchase

https://www.roughlyexplained.com/2017/05/white-house-counsel-don-mcgahns-uncle-helped-trump-deal-mob/

Because it would look bad for Trump to working with hitmen. His lawyer found a solution.

New Jersey state investigative records show that Trump paid $1.1 million for the site, after the title was transferred from Testa and Narducci to Patrick McGahn’s secretary and then to a Trump entity.

If that name McGahn sounds familiar? That's because he was the uncle of Trump's White House Counsel Don McGahn.

So Trump is closely working with mobsters and the NJ DGE won't give him a license if he is working with mobsters.
So what does he do? He lies. He tries to tell everyone what he thinks they want to here.

and that is what people don't get about the document you linked to.

It's a Cover Your Ass document.

The FBI agent wrote this document after the Special Agent in Charge of the main NJ FBI office called him up and Do you want to tell me what in the hell is going on? I just got a call from the NJ DGE asking about one of confidential informants? WTF did you do?!

Trump tried to the get the DGE off his back by telling them that the FBI vouches for Sullivan. So the DGE calls the FBI and say Donald Trump is telling you think Dan Sullivan is clean. So the SAC calls this guy and says you need to put out this fire. So this guy now has to write a memo explaining everything that happened.

There are two parts of this that absolutely kill me.

Trump playing dumb telling the FBI he read in the newspapers that organized crime is involved in Atlantic City! In the newspapers! Meanwhile he's going around bragging that he's working with Daniel Sullivan. "He's the guy that killed Jimmy Hoffa," Trump would tell people. His lawyer Roy Cohn is the lawyer for Fat Tony Salerno and Paul Castellano. The heads of the two most powerful mafia families in the country. Both of them owned a concrete company that Trump was buying his concrete from. Shocked! I'm shocked https://youtu.be/SjbPi00k_ME?t=9

The second part is Trump trying to get the FBI to agree to the lies he told the NJ DGE. Again by telling them what he thinks they want to here. I'm concerned about my repuation I'll let you do an uncover operation in the casino......LOL....THERE's NO CASINO at this point. It's September 1981. He won't even finalize his deal with the hitman for the parking lot until 1982. The Casino doesn't open until 1984. To use an east coast word, it's a fugazy, it's a check that is going to bounce.

When the FBI guy says this is nuts, I can't promise you anything, Trump tries to find out who he could talk to. Why because without that casino license he will never open a casino. And the DGE tells Trump we spoke to your business partner, Sullivan, and he was hiding his background and his business activities. So Trump tells the DGE, look talk to the FBI, Then the FBI guy has to tell his boss that Trump told the DGE the FBI would vouch for him and we can "nip this thing in the bud."

On Sept. 21, 1981, the FBI got a strange call from Trump. He said he had traveled to Trenton the previous week to meet with Mickey Brown, the director of New Jersey’s Division of Gaming Enforcement. Trump was worried about the status of his application for a casino license. Brown told Trump that everything was on track except for “one problem” that might draw out the investigation — his ties to Sullivan, according to an FBI report.

Brown said that Sullivan had not been candid with investigators about his background and his business activities.

To defend himself and “nip things in the bud,” Trump said he told Brown that Sullivan had introduced him to two FBI agents and was close to the agency.

And to be clear. Trump's offer to the FBI was, in fact, a fugazy. There never was an undercover operation. It never happened.

By late September, the FBI proposal was in a “thoroughly finished state,” but, Stowe said, it apparently never came to fruition.

So how did Trump get his casino license?

A. He got out of some of his businesses with Sullivan including the drywall company

B. He lied to the NJ DGE about his other dealings with Sullivan

Under pressure from regulators, Trump “advised the Division that he would not have any future personal, social or business dealings with Sullivan other than in the context of their Atlantic City lessor-lessee relationship.”

Despite what he told gaming officials, Trump stayed in touch with Sullivan. In early 1982, he tentatively offered Sullivan a job as his organization’s chief labor negotiator, with a $75,000 salary, according to a civil lawsuit Sullivan filed against New Jersey authorities in 1983.

Trump having to get out of the drywall company turned out to be a stroke of luck

Around that time, Trump took a step that would complicate the gaming review. He agreed to finance Sullivan’s purchase of Circle Industries, a drywall manufacturer that employed Sullivan.

Sullivan told Stowe and another agent about the Circle acquisition plans, according to a report they wrote on May 7, 1981. It said that Sullivan, Trump and a third man, then president of Circle, “will be equal, one-third partners,” with “Sullivan overseeing labor relations and Trump financing the enterprise.”

Circle Industries was among a group of some 20 drywall makers that secretly referred to themselves as “the club.” The companies held significant power in New York’s construction industry because of the importance of their product in so many projects.

Because the feds went after the drywall club in 1983. And the company Trump was involved with was pat of this cartel. https://www.nytimes.com/1983/07/06/nyregion/investigators-charge-contractors-formed-club-for-tax-fraud.html

Investigators identified some of the companies under investigation as Ace Partitions Inc., 2216 Bath Avenue, Brooklyn; the Circle Industries Corporation, 4817 Baldwin Avenue, the Bronx;

Sauces Sullivan and Shapiro and Cohn and Fat Tony https://www.thechicagosyndicate.com/search/label/Kenny%20Shapiro?m=0

The FBI agent who wrote the memo https://www.phillyburbs.com/story/news/2016/09/23/trump-s-ties-to-informant/18154546007/