Source: A Little Leitgeb Research, from the Michael Lorenz blog
That "the horn player Leutgeb was a cheesemonger in a suburb of Vienna" is a popular myth that persistently refuses to die ("Blessed are the cheese-makers, for they shall have Mozart horn concertos."). It is based on a number of misunderstandings, aggravated by lack of archival research. Leitgeb's first father in law Biagio Placeriano was born around 1686 in the Friulian village of Montenars (he is related to the Italian author Francesco Placereani). The presence of Placeriano's older brother Antonio (also a cheesemaker) in Vienna is documented as early as 1724, on the occasion of his marriage to Theresia Gull in Liechtental. Biagio seems to have accompanied or followed his brother to Vienna where he also worked as "Welischer Käßmacher" (Italian cheesemaker). A "travelling cheesemaker" named Jakob Placeriano (maybe third brother) died on 1 May 1771 in Vienna. On 3 February 1732 Biagio married Catharina Morelli, the daughter of his landlord in Altlerchenfeld, the bellows maker Nicolaus Morelli. In Morelli's house "Zum heiligen Geist", Altlerchenfeld No. 42 (today Lerchenfelderstraße 160, a building torn down in 1881) Placeriani established a shop where he produced Italian sausages and cheese.
It is important to note that Placeriano was not a regular cheese maker (a profession classified as "Kässtecher" in 18th-century Vienna), but a so-called "Cerveladmacher" (also "Servaladawürstmacher"), which means that he produced various sorts of Italian cured meat sausages (Salumi) and Italian hard cheese, such as Parmesan. Biagio Placeriano died on 16 October 1763 of lung gangrene.
For a short time his widow kept the cheese shop going, but in 1764 sold the "Cerveladmachergerechtigkeit" (the sausage making license) to a certain Johann Rotter (misspelled "Rotta" in the records). Her horn playing son-in-law Joseph had nothing to do with all this. Between March 1763 (after his unsuccessful employment at the Esterházy court) and 14 September 1763 (the date of birth of his son Johann Anton) he had moved to Salzburg and joined the chapel of the archbishop.
Of course the blame for the origin of the "cheese shop myth" lies with Leitgeb himself. On 1 December 1777 Leopold Mozart wrote to his son in Mannheim:
H: Leutgeb, der itzt in einer vorstatt in Wienn ein kleines schneckenhäusl mit einer kässtereÿ gerechtigkeit auf Creditgekauft hat, schrieb an dich und mich, kurz nachdem du abgereiset, und versprach mich zu bezahlen mit gewöhnlicher voraussetzung der Gedult bis er beÿm käs=Handl reicher wird und von dir verlangte er ein Concert.
Mr. Leutgeb, who has bought on credit a snail's shell with rights to a cheese business in a suburb of Vienna, wrote to us after you left and promised to pay me with the usual implication of patience until he will get richer trading cheese and from you he requested a concerto.
Viennese archival records such as tax registers and the 1788 Steuerfassion however show that Leitgeb never ran a cheese shop. Since it is highly unlikely that he had the expertise and the necessary business prospects to actually become a cheesemonger it seems that this cheesemaking story only served as part of a scheme to elicit money from Leopold Mozart. When in 1777 Leitgeb and his wife bought the house "Zur Heiligen Dreifaltigkeit" ("At the Holy Trinity", built in 1748, today Blindengasse 20) at an auction from the furrier Anton Ditzler, they had to borrow the larger part the money from Ferdinand Aumann, a butcher in Penzing. On 1 July 1778 Leitgeb already had to mortgage the house at a four percent interest rate. In 1783 the mortgage was transferred to a certain Joseph Aufmuth and was only discharged in 1812, after Leitgeb's death. It is very unlikely that Leopold Mozart was ever paid back the money he had lent to his former colleague musician.