The 173m² apartment is located in a 17th-century town house on the Seineside Quai de Conti in the fashionable sixth arrondissement and has palatial four-metre-high rooms.
It recently sold for a record price per square metre for the French capital - about 42.000 euros, compared to17,000 euros in another prestigious area, Avenue de Montaigne in the swish eighth arrondissement and five times the Parisian average.
The previous owner billionaire Francis Holder, who owns bakery chain Paul and macaroon-maker Ladurée, bought the property from Paris city council in 2007 for 3.1 million euros, according to economic monthly Challenges.
Five years later Holder put it up for sale via real estate agents Philippe Ménager and Nicolas Hug, which specialises in luxury properties.
The agency conducted negociations between Holder and the Austrian businessman who has bought the flat for a year without concluding a sale and is now claiming a 400,000-euro fee and compensation.
"The purchaser waited for our mandate to expire," Philippe Ménager told RTL radio, "and agreed the sale behind our backs to avoid paying fees, which are actuallly negligeable in relation to the price of the apartment."
Holder's wife, Françoise, denies there has been an illegal agreement.
"The mandate stipulated that the fee was payable by the buyer," she told Challenges, adding that she doubts that the agency will not receive its fee.
In 2007 Holder sold another apartment via the Ménager and Hug agency - a 1,000m² flat, originally bought 7.6 million euros in 2002 and sold for 27.8 million euros five years later.
Both billionaires have been summoned to court to answer the estate agents' claim.