Hello. You are right about Dumas, he accessed a lot of archival material that was destroyed. But he also made a few things up.

However, when it comes to a safe conduct then, yes, there does seem to have been such a thing. Ruffo informed Micheroux, one of his subordinates, that “I have the letter of guarantee, and that from Sir Hamilton in the name of Admiral Nelson. So Commandant Massa [the rebel] can depart in good spirits; we are incapable of deceiving him and Nelson, whilst before he didn’t wish to permit it, has now spoken clearly. Added to this we have what it says at the end of Lord Nelson’s letter and the other one written by Captain Ball. You can show the one or the other to Massa and keep them for our records."

Micheroux went into the castles with captains Troubridge and Ball and informed Acton in Palermo that:

"His Eminence wrote to me that Lord Nelson consented to put the capitulation into effect, and that I would have to bring the Russians back to the abandoned positions. As proof His Eminence urgently sent over the enclosed documents as guarantees to the garrisons but, as they trusted my word of honour, I did not need to make use of the documents."

So they certainly existed. Trouble is they disappeared. I add some more proof that the rebels were told they could leave the forts and sail in the book.