The battle between two major players in the art world, Yves Bouvier, a businessman who sold $2 billion worth of art, and Dmitry Rybolovlev, an oligarch who bought it, took a turn in Mr. Bouvier’s favor Thursday as a Monaco court dropped criminal charges against him, asserting the prosecution of him had been unfair.

The quarrel between the men dates back several years to when Mr. Bouvier helped Mr. Rybolovlev, a Russian billionaire mining magnate, buy 38 pieces of world-class art for a startling $2 billion.

Mr. Rybolovlev has said that Mr. Bouvier acted as his agent and adviser on the transactions and earned fees for that service. But he later discovered, he said, that Mr. Bouvier had bought many of the items in advance, then flipped them to him. He accused Mr. Bouvier of overcharging him by about $1 billion.

A judge in Monaco’s Court of Appeal tossed out the charges of fraud and money laundering, after concluding that the investigation of Mr. Bouvier had been “conducted in a biased and unfair way without the defendant being in a position to retrospectively redress these serious anomalies that permanently compromised the balance of rights of the parties,” according to excerpts from the decision provided by Mr. Bouvier’s representatives. The decision was confirmed by Mr. Rybolovlev’s representatives.