The letter dates from Napoleon's ill-fated invasion of Russia in the early 19th Century, and is written in code to his foreign minister Hugues-Bernard Maret in Paris in 1812.
The first line of the letter reads: "On the 22nd at 3am I will be blowing up the Kremlin".
It is being sold alongside a deciphered transcript.
Paris-based Museum of Letters and Manuscripts was finalising the purchase of the document for 187,500 euros (USD 243,500), including fees, media reports said.
Earlier, the letter was expected to fetch up to 15,000 euros (USD 19,000) at an auction in France.
In the letter, the French emperor reveals his frustration at the Russian campaign, with his army ravaged by disease, cold and hunger, and already in retreat from Moscow.
"My cavalry is in tatters, a lot of horses are dying. Make sure we buy more as soon as possible," Napoleon also wrote.
Napoleon kept the promise to blow up the Moscow Kremlin, destroying the Kremlin's walls and towers before retreating with his army, beginning a decline in his power that would lead to his abdication and exile just two years later.
Jean Christophe Chataignier, director of the Osenat auction house where the letters will go on sale, said the coded letter was "very rare" and was expected to fetch between 10,000 and 15,000 euros.
Napoleon, who died at the age of 51, was Emperor of France from 1804 to 1815.
