The stamp, which was printed erroneously in blue instead of orange, was the centre piece of the four-day auction by Galerie Dreyfus which ended on Friday.
It had been expected to fetch between €1 and 1.2 million.
According to the auction house catalogue, there are only two known examples of this error of colour; the one sold is the only one with a date stamp. The other was removed from the same envelope and kept by the collector in Syracuse who originally discovered them, and who sold the letter with the remaining stamp to a dealer in Paris. The catalogue does not say when this happened.
The auction house said the stamp was sold on Thursday to an on-line bidder in the United States, who was described by a spokesman as “based in France”.