Professor Robert Black was key architect of the Lockerbie trial held under Scots law in the Netherlands but has since become a campaigner for the release of Abdelbaset Ali Mohmed al-Megrahi.
The QC, a professor emeritus of Scots law at Edinburgh University, is demanding £470,000 in unpaid fees for legal advice given to Megrahi’s lawyer, Ibrahim Legwell.
Prof Black had joined Lord Trefgarne, a former trade minister in Margaret Thatcher’s government, in providing political and legal expertise.
Details emerged in a letter to the son of deposed Libyan leader Colonel Gaddafi, which was found in a house near the capital Tripoli.
Lord Trefgarne wrote to Saif al-Islam in 2007 asking for help to recover the fees.
He wrote: “The amount now owing following a small payment is $1.5 million. Prof Black and I would be very grateful for your advice in this matter.”