Displayed on the sidelines of the Salzburg Festival in Mozart's birthplace, the 12-page manuscript is a transcription in the young composer's hand of the Stabat Mater -- a Marian hymn -- by Italian master of counterpoint Eugenio di Ligniville.
The manuscript, sold at the London auction in May by a private individual, also bears annotations by the child prodigy's father Leopold.
Experts date the manuscript to 1773, when Wolfgang Amadeus Mozart was 17.
"It is one of the most impressive single manuscripts arising from Mozart's concentrated canonic and contrapuntal studies in Salzburg between 1772 and 1774," Sotheby's said in its description of the lot.