Fereydoun was arrested on Sunday for allegedly receiving zero-interest loans and influencing the appointment of a bank director who Rouhani was forced to dismiss over his enormous salary.
Conservatives have demanded Fereydoun be put on trial.
Fereydoun's arrest came amid a major probe that had led to the detention and imprisonment of "other people", judiciary spokesman Gholamhossein Mohseni Ejei Ieri told reporters on Sunday.
"Yesterday, bail was issued for him, but because he failed to secure it he was sent to prison," Ejeie said, adding that Fereydoun would be released if he secured bail, pending trial.
Formerly Iran's ambassador to Malaysia and a member of the Iranian delegation to the UN, Fereydoun has acted as a key aid to Rouhani, who was re-elected in May.
Fereydoun was also involved in negotiations between Iran and Western powers that led to a landmark nuclear deal in 2015. The deal enraged the country's conservatives, who control the judiciary.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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