The Iranian scientist claiming to have been kidnapped by the US provided the Americans with information for years before his disappearance last year, the US media has reported.
Unnamed US officials told reporters Shahram Amiri was paid $5m (£3.3m) for “significant, original information”.
Mr Amiri flew back to Tehran from the US on Thursday to a hero’s welcome.
He disappeared while in Saudi Arabia on a Muslim pilgrimage last year, and resurfaced in July asking to go home.
The reports by the New York Times and the Associated Press both quoted unnamed US officials who wanted to counter Mr Amiri’s assertions that he had been tortured by the CIA while in the US.
“His safety depends on him sticking to that fairy tale about pressure and torture,” the New York Times quoted the unnamed official as saying.
“His challenge is trying to convince security forces that he never cooperated with the United States.”