It is a pity that the few sources covering his passing are choosing not to remember his many good qualities, as is the norm in obituaries and death notices, but instead are dragging up old dirt. In some ways his 'exotic' Middle Eastern roots, which are the reasons for this bigotry, are also the reason so many dealers in the West tried to take advantage of him - and then make ridiculous claims to besmirch him once they were caught.
Yes many dealers presented Al-Thani with hugely over-inflated invoices (see here), and Al-Thani was briefly under house arrest in Qatar whilst those were investigated in 2005. But he was cleared, and the various dealers ... well put it this way, many of those dealers are now not going anywhere near the lucrative Qatari art market for fear of being arrested.
My personal experience of Al-Thani prior to this period was that he was willing to perhaps over-pay a little to acquire top pieces, and may not have fully understood export restrictions, but he was not the one being dishonest.
More recently, there were accusations made about him when he refused to pay for ancient coins bought at one auction:
One of the world’s top collectors Sheikh Al-Thani dies suddenly, aged 48 - The Art Newspaper:
In 2012, a High Court judge in London froze $15m worth of his assets as part of a dispute over unpaid bills to auction houses. The numismatic auctioneers Baldwin’s, Dmitry Markov and M&M Numismatics, accused him of defaulting on bids for items from the Prospero Collection, a cache of Greek coins.The case was settled out of court, but the reason he refused to pay was because of persistent rumours that the prices were again inflated, and that the auction house had put together the collection and failed to declare they were the owners. These rumours may or may not be true, and because the case was settled there was no further investigation either way - but it is worth pointing out that there were again two sides to this story, even though only one used PR methods to make theirs appear in the press.
Last year I came across an even more ridiculous story. An art bounty hunter trying to win the business of Israel suggested that Al-Thani and his cousin were holding the stolen Samaritan Torahs. By 'holding' in fact he was claiming they were offering to ransom them back to Israel (a rather tall tale, since the Samaritans are at Nablus in Palestine, but when dodgy people try to get lucrative clients to sign them onto a retainer, they'll say any nonsense).
The art market used to be pretty anti-Semitic even though there are many Jewish artists and collectors. It saddens me that they have transferred this racism to Arab clients, and make such accusations when they get caught with their fingers in the cookie jar.
Al-Thani was an interesting, erudite man and the art world will be a sadder place without him.