"The case went in the wrong direction from the very beginning. There were many manipulations in the original files, many errors ... This is why we should seek a compromise," al-Islam said, adding Tripoli had already discussed a plan with Germany and France.
Al-Islam runs a charity foundation which has played a key part in negotiating with Western countries over compensation for bombings in which Libya was implicated.
Experts have said the six may escape the firing squad, with a government-led body having the final word on their fate.
Feim Chaushev, Bulgaria's deputy foreign minister, said al-Islam's comments could be a sign of progress in negotiations.
"I am inclined to see a positive signal in these assurances," he said.