William Casey Confirmed the October Surprise (from Secrets and Lies: A History of CIA Mind Control and Germ Warfare). In a conversation with CIA agent William Buckley (p317):

Casey had told Buckley the story of the hostages release. Known as the "October Surprise," it was an operation that Casey had orchestrated over dinner one night with some of the future president's aides. Casey had raised the possibility of doing a deal with Iran and said there would "have to be something in it for the ayatollahs." He had proposed approaching a number of wealthy Republicans. In a week they had pledged $40 million. The money was to be laundered through a number of Swiss banks before ending up in Tehran. Earl Brian, a business associate of Reagan, agreed to handle the transfer because of his long-standing high-level contacts in Tehran. He arranged to fly to Paris on October 17, 1980, less tha a month before the race for the White House was decided. His cover story was that he was going to sign a deal for one of his own companies. The next day, sixteen high-ranking Iranian government officials arrived. It took two days to hammer out the deal. Terms for the hostage's freedom were agreed on.

One of the terms, of course, was that they not be released before the election.