Green told the detectives were initially sceptical that 50 Cent was supporting Hassan’s bail application, which the judge turned down.
“When you get something like that come in, it’s quite a unique moment and I think the team were a little bit shocked,” he said.
“You obviously think it’s fake to start with… it’s obviously one of those things you’ll never forget as part of an investigation.”
But detectives say Hassan’s defence team later submitted photos and videos that showed him on private jets and at US tour dates.
The rapper also provided another letter for the sentencing hearing as part of Hassan’s mitigation.
In it, 50 Cent said he had “known and worked closely with him for many years, both professionally and personally”.
The rapper said in the letter: “I write as someone who has relied on him [Hassan] directly in my own business and who knows his character through years of firsthand experience.
“I am aware of the matter that brings Mr Hassan before the court, and I do not write to minimize its seriousness. I write only to give the court an honest account of the man I have come to know through working alongside him over the years.”
Later in the letter, 50 Cent said “employment will be available” to Hassan upon his release, adding: “His experience, reliability and understanding of the work make him someone I would welcome back without hesitation.”