The Sarkozy campaign received a total of 4.5 million euros from Libya in 2007, Saif al-Islam says in a letter dated 11 July 2018, according to Le Monde newspaper.
The allegations are not new - Sarkozy was indicted for passive corruption, illegal campaign finance and receiving misappropriated public money in relation to the Libya case in March.
Saif al-Islam Kadhafi first made the accusation in 2011, as his father's regime was in the process of being toppled by Western-backed militias, leading to the French investigation.
The letter does not provide proof, although Saif al-Islam insists he has it, but it does add some lively detail to the account.
Sarkozy's messenger, Claude Guéant, had to stand on the suitcase containing the first payment of 2.5 million euros to close it because it was so full of cash, the letter says, citing Bashir Saleh Bashir, the Kadhafi aide who allegedly handed it over as the source of the anecdote.
Guéant, who was later to become interior minister, told Le Monde he has "never seen a centime of Libyan money" and accused Saif al-Islam of being "consumed by a thirst for vengeance" because Sarkozy was the "principal architect of the military intervention in Libya" that brought down his father.
During the course of the investigation Guéant at first denied the existence of large sums of cash, then said they were donations from French supporters.
Saif al-Islam's letter also claims that a further two million euros were paid to the Sarkozy campaign in return for a promise to pardon Kadhafi-era intelligence chief Abdallah Senussi, whom a French court had sentenced to life imprisonment in his absence after ruling he had been involved in the bombing of an airplane that killed 170 people in Niger in 1989.
The Libyans also discussed with aides to Dominique de Villepin, whose hat was in the ring to become the right-wing presidential candidate in 2007, but concluded that Sarkozy was more likely to win, the letter says.
Franco-Algerian Alexandre Djouhri, who was the intermediary in those negotiations, later switched to the Sarkozy camp.
He visited the Elysée presidential palace 73 times between 2007 and 2011 - 59 times to see Guéant and 15 to see Sarkozy - sometimes accompanied by Saleh, according to L'Obs magazine.
Djouhri denies ever having worked with Bashir Saleh Bashir but Saif al-Islam claims that he threatened Saleh with death if he ever went public about the campaign finance.
Djouhri is currently in London, awaiting a hearing in November that will decide whether to hand him over to the French authorities.
Saleh, who was smuggled out of Libya, is currently in the UAE, receiving medical care after being shot in an apparent carjacking in Johannesburg, where he was living.
Saif al-Islam Kadhafi was captured by a militia in 2011 and, according to the letter, kept in solitary confinment until 2015 when he benefitted from an amnesty declared by one of the country's two rival parliaments.
His current whereabouts are unknown and his mental health may have been affected by his detention and torture, a former Libyan official told Le Monde.
Pressure to sign nuclear deal
In further revelations Tuesday, former Areva boss Anne Lauvergeon is said to have told the investigation that an agreement in principle on building a nuclear power station in Libya was signed by then foreign minister Bernard Kouchner in July 2007 without consulting her and with "unusual rapidity".
Shortly after being elected, Sarkozy phoned Kadhafi to say he was ready to send an exploration team on the question, according to Libya's diplomatic service.
But Areva management declared that Kadhafi's regime was "not sufficiently rational" to handle a civil nuclear programme, a reversal of roles according to Lauvergeon.
"Classically, it is companies who are mercantile and ready to sell in spite of the risks and governments are there to recall them to reason and remind them of the limits," Lauvergeon said, according to the Mediapart website.
A meeting in a Paris hotel was organised with a Libyan nuclear official, with one of Kadhafi's sons dropping in at one point, but Areva did not change its mind.
After the company's refusal to pursue the deal, relations with the presidency seriously deteriorated, Lauvergeon told the investigators.
Guéant reportedly raised the question again in 2010 in a meeting with Lauvergeon and Henri Proglio, who was then head of power company EDF.
Given the civil war that currently rages in Libya, "we can only be glad that we didn't start building reactors there", Lauvergeon comments.
After she published a book making the same claims in 2012, Sarkozy called them "grotesque" and said that no such plans existed.