The assistant, who owed Saleh tens of thousands of dollars, was on a reimbursement plan, said the official.
Saleh, 33, was found dead Tuesday in his Lower East Side apartment, police source told CNN
When Saleh's sister rang at her apartment, the assistant allegedly dismembered her body with an electric saw and fled, according to the official.
He was last seen in a surveillance video entering an elevator in his building and a man dressed in black - who, according to the police, is Saleh's attacker - entered the elevator with him, according to the law enforcement official.
The elevator in the Saleh building goes directly to the apartments, the source said.
Saleh died of neck and torso injuries; the way of dying was homicide, according to the medical examiner.
Saleh, the son of Bangladeshi immigrants, was the founder and CEO of Gokada, a motorcycle app in Nigeria.
He recently founded venture capital firm Adventure Capital - which has invested in carpooling start-ups in countries like Bangladesh and Colombia.
"Fahim is more than what you read"
Saleh's family recalled his "bright and innovative spirit" in a statement confirming his death this week.
"The headlines speak of a crime that we still cannot understand," the statement said. "Fahim is more than what you read. He is so much more. His brilliant and innovative spirit took everyone in his world on a trip and he made sure to leave no one behind.
"There are no words or actions to comfort us, except the capture of the person who has shown nothing wrong to our loved one."
While still in high school, Saleh founded PrankDial.com, a website for pre-recorded phone calls which, he wrote in 2018, had generated more than $ 10 million since its inception. He continued to build and sell sites throughout his adolescence and during his stay at Bentley University in Massachusetts.
Over the years, Saleh has sought to boost the careers of other young people in the tech industry.
On Medium, where he blogged regularly, Saleh called his Gokada foundation "one of the most" things he has ever done. His limited knowledge of Nigeria's transportation system initially discouraged the Nigerians he attempted to recruit for his start-up.
In 2019, Gokada raised $ 5.3 million in seed funding and hired more than 800 drivers, but Gokada's business experienced an unexpected downside in February when the state of Lagos banned commercial motorcycles in the city .
Saleh videotaped a passionate call on behalf of its employees to lift the ban as the company pivots on delivery and works to launch a hail service for ships, he told CNN in February.
