Iran's semi-official Fars news agency meanwhile said Zakka would be handed over to representatives of Hezbollah on Tuesday, rather than the top Lebanese security official who flew to Tehran earlier this week.
Nizar Zakka, who has US residency, flew to Beirut with Lebanon's security chief Abbas Ibrahim, the official told Reuters.
Zakka, an information technology expert and permanent U.S. resident in Washington DC, disappeared in Tehran while attending a state-sponsored conference in September 2015.
Although Iran has recently portrayed Zakka's case as a matter between Iran and Lebanon, Iranian Foreign Minister Javad Zarif in 2016 said it was a problem mainly between Tehran and Washington. He denied the charges, while Iranian media have described him as a US spy. Both the Congress and the Senate passed resolutions in 2017 calling for the unconditional release of U.S. citizens and residents detained in Iran, including the Lebanese national.
Freed Lebanese businessman Nizar Zakka, who had been detained in Iran since 2015, gestures during a news conference at the Presidential Palace in Baabda, Lebanon June 11, 2019.
Zakka flashed reporters the victory sign and hugged his brother Ziad before he went into the meeting with Aoun, who had personally requested his release.
Mr Zakka was arrested in September 2015 after being invited to speak at a conference hosted by the Iranian government.
The potential release of Zakka, an internet freedom advocate, comes as tensions between Iran and the U.S. remain high.
"Some local media has speculated that Zakka's release would also involve the freeing of another Lebanese citizen in the United States".
A top Lebanese security official is in Tehran to secure Zakka's release, which has been anticipated.
In 2016, he was sentenced to ten years in prison and ordered to pay a fine of $4.2m for espionage.
Zakka's IJMA3 organization had received at least $730,000 in contracts and grants since 2009 from both the State Department and USAID, the lead American government agency fighting poverty and promoting democracy across the world. "We did all we could to stop this from happening, but we are seeing that we have failed to make a significant impact".