Defending his part in the negotiations, Straw said: "I'm unapologetic about that ... Libya was a rogue state. We wanted to bring it back into the fold.
"And yes, that included trade because trade is an essential part of it and subsequently there was the BP deal."
However, Straw's spokesman insisted that al-Megrahi's release was not agreed on because any possible transfer was always subject to a veto by the Scottish authorities.
Al-Megrahi was the only person convicted of the 1988 bombing of Pan Am flight 103 over the Scottish town of Lockerbie which killed 270 people. He was released last month on compassionate grounds because he is terminally ill.
Previously, London has been forced to deny allegations that it struck a deal with Libya to free him in return for improved trade ties.
Gordon Brown, the British prime minister, announced on Thursday that "there was no conspiracy, no cover-up, no double dealing, no deal on oil, no attempt to instruct Scottish ministers, no private assurances by me to Colonel Gaddafi".
But documents released this week show that Straw used the possible release of the prisoner when talks on the ratification of a huge oil deal between BP and Libya had become bogged down.
The $900 million deal was ratified in January 2008 shortly after Straw's change of strategy.
But Musa Kusa, Libya's foreign minister, said in an interview on Saturday that trade had nothing to do with al-Megrahi's release.
"You should not do an injustice to the British government," he told British newspaper The Times from Tripoli. "It was nothing to do with trade. If we wished to bargain we would have done it a long time ago."
BP said Friday it had lobbied Britain to speed up the agreement with Libya to improve business relations, but denied pressing for al-Megrahi to be released.