0644 GMT July 21 2017
"Yesterday, bail was issued for him but because he failed to secure it he was referred to prison," Deputy Judiciary Chief Gholam-Hossein Mohseni-Ejeie said in a televised press conference in Tehran.
Ejeie, who is also the judiciary spokesman, added that Fereydoun would be "released once he provides the bail”.
The spokesman did not give any details about the charges and the sum of the bail.
The arrest comes in the wake of President Rouhani’s landslide victory in May presidential election in which he routed his conservative rivals.
Rouhani’s supporters argue that the conservatives try to mount pressure on him by imprisoning his brother and take other measures against his policies.
"Multiple investigations have been conducted regarding this individual, also other people have been investigated, some of whom are in jail," Ejeie said.
Fereydoun has acted as a top aide and advisor to Rouhani, who changed his surname from Fereydoun years ago. He was also Rouhani’s representative in Iran’s negotiating team at marathon talks that resulted in a landmark nuclear agreement with world powers in 2015.
Conservatives have demanded Fereydoun be put on trial, accusing him of receiving zero-interest loans and influencing the appointment of a bank director who was dismissed for taking an enormous salary.
He was previously accused of having links with the officials at the center of a scandal involving inflated salaries for managers at the state insurance firm. He has always denied any wrongdoing.
A couple of weeks ago Fereydon lodged a complaint to a board supervising the conduct of lawmakers against an MP, who had accused him of having a role in some economic and banking issues.
After hearing the MP’s arguments, the board was not convinced and asked the legislator to be accountable to the judicial officials.