The documents of Mozart’s life — letters, memoirs of friends, portraits, bureaucratic files — have long been scrutinized at a microscopic level. So when his name was discovered two decades ago in a Viennese archive from 1791, it caused a stir.

The archive showed that an aristocratic friend and fellow Freemason, Prince Karl Lichnowsky, had sued Mozart over a debt and won a judgment of 1,435 florins and 32 kreutzer in Austrian currency of the time (nearly twice Mozart’s yearly income) weeks before the composer died....

Now a Mozart scholar, Peter Hoyt, has come up with a theory about the details: that the judgment stemmed from a loan of 1,000 thalers in Prussian currency made on May 2, 1789, the day the prince and his coach departed Berlin without Mozart, leaving him in need of money for his own transportation....