A Russian billionaire has lost a £36m (US$54.2m) deposit he paid to buy the most expensive house in the world on the French riviera, plus another £1m interest. Mikhail Prokhorov - the world's 24th richest man - offered £360m for the sprawling Villa Leopolda in 2008. After making a 10% down-payment, he then backed out of the sale after the global credit crunch hit. But under French property law, once an initial sale contract has been signed, a deposit can only be refunded during a seven day 'cooling-off' period. A court in Nice ruled yesterday that the villa's owner, 71-year-old Lily Safra, could keep the £36m. A judge also ruled that Prokhorov, 44, must pay Safra an additional £1m in interest. The Villa Leopolda in Villefranche-sur-Mer was built for King Leopold II of Belgium in 1902.