D'Antraigues Was Also Offered an Arabic Text of Livy?

Avant la fin de l'année, noire voyageur, ayant rassasié sa curiosité à Constantinople, la porta en Egypte. D'Alexandrie au Caire, du Caire à Suez et au Sinaï, il la dispersa sans l'épuiser, auprès des pachas qui gouvernaient le pays et dont il eut audience ; sur le Nil, qu'il remonta en bateau jusqu'à Antinoopolis, et où il fut arrêté par des partis armés, sans pouvoir atteindre Thèbes; dans les villes et les villages, qu'il parcourut en observateur attentif aux moindres détails de moeurs; dans les couvents, où il cherchait quelque manuscrit précieux, et où il affirme avoir découvert et inutilement offert d'acheter un Tite-Live complet en arabe, ainsi qu'un Diodore de Sicile également complet; au pied des Pyramides et des autres monuments de la vieille civilisation Egyptienne, qu'il contempla avec le désir de contrôler les assertions des voyageurs anciens et modernes, 'Hérodote et de Thévenot. Le Marseillais Magallon, lors sans titre le principal représentant de la France dans ces contrées, l'accueillit avec empressement et lui servit çà et là d'interprète et de guide. L'Egypte était déjà à la mode en France, et d'Antraigues conquit sur le Nil cette réputation géographique qui devait précéder dans les salons parisiens, où l'hellénisme faisait concurrence à l'américanisme, sa réputation politique. [Un agent secret sous la revolution et l'empire : le comte d'Antraigues (1893) p. 22]

Apparently the existence of this text was widely reported by European travelers.  D'Antraigues would represent the earliest citing of the Arabic text of Livy:

By the end of the century there was again talk of the existence of an Arabic translation of the complete Livy somewhere in Morocco. One scholar even tried to get the Austrian government to send for it. Apparently it was a variant of this same which Niebuhr told his classes in 1828-29 when he referred to the alleged discovery of an Arabic version in Saragossa, Spain.

With all this background, it is easy to understand the interesting happenings at Palermo in 1782. In that year a certain Joseph Vella, an Italian born in Malta, acted as interpreter for a distinguished Arab from Morocco who was forced by bad weather to put in at Palermo. There, among other things, the visitor was shown a number of Arabic manuscripts. The interpreter became interested in them and took it upon himself to read and explain them and thus to parade his erudition. But his inventiveness far exceeded his knowledge, and to save his face and to maintain the interest which he had aroused he came to depend entirely on his imagination. Soon he announced that he possessed an Arabic translation of Books LX-LXXXVIII of Livy's history, which had been given to him by the Grand Master of Malta. The latter, he said, had received it from a Frenchman who took it away from a thief in the church of St. Sophia in Constantinople. International intrigue and crime such as are found in the pages of a mystery thriller. Vella did not publish the new Livy, though an English lady offered to defray the expense. In 1790 it was reported that the lost books had arrived in Dublin from Palermo. This was in a letter appropriately sent to Bishop Percy, who in his famous Reliques published a collection of ancient English poems the manuscript of which he saved from the fire to which a housemaid was about to consign them. Finally Vella issued an Italian translation of the supposed Book LX. But the entire publication consisted of a single page, for he had simply translated the well-known Epitome. After this apprenticeship he went into the forgery business on a grand scale, specializing in mediaeval letters and documents. He achieved considerable fame and wealth thereby, was made abbot and professor of Arabic. He published numerous expensive volumes out of funds which were provided by others, and some of these were translated into Latin, English, French, and German. Finally in 1794 he pretended that he had been robbed of his precious manuscripts. Testimony showed that he had shipped a large box the day before the supposed robbery. From a study of the published works, a German scholar, Hager, denounced him as a fraud and then proceeded to Palermo for further investigation. After two After two years of study, Hager set forth such a convincing array of evidence against Vella that the latter confessed and was sentenced to fifteen years' imprisonment. [Ullman, the Post-Mortem Adventures in Livy p. 69 - 71]

D'Antragues claims seem to have been the inspiration to the story.   Who else but D'Antragues could have been the "Frenchman who took it away from a thief in the church of St. Sophia in Constantinople."  D'Antragues travels through the Orient began with his stay in Constantinople to visit his uncle François-Emmanuel Guignard, comte de Saint-Priest who was the Ambassador and a member of the Order of Malta.  An older retelling of the story here. 

Indeed there are several versions of the story where the ambassador is named:

Mr Giardin, the French Ambassador at Constantinople, has sent to Paris fifteen valuable works in Arabic from the Imperial Library at Constantinople, among which are the complete works of Plutarch and Herodotus

Clearly, 'Giardin' (= Pierre de Girardin ambassador from 1686-1689) is a mistake for François Emmanuel Guignard who was ambassador from 1768-1784. 

Here is the closest to original account I can find, that of Hager, the man who proved Vella to be a fraud:

Vella, pursuing the same idea [i.e. the existence of the Arabic text] speaks of a French painter (Fabre or Favay)  who, as he was drawing the mosque of St. Sophia, at Constantinople, had found on the cornice of that magnificent fabric a packet, which he took possession of, and which he presented to the Grand Master, Pinto, at Malta.  He is said to have presented this valuable relique to his favourite, Vella, with the following words : " Take this manuscript, it will some day render you happy !"