The Swiss art dealer Yves Bouvier, known as the “freeport king,” is under investigation for tax evasion in Switzerland. He could be liable for as much as CHF 165 million ($145 million) in back taxes including debts on proceeds from the sale of 37 artworks he brokered on behalf of Russian billionaire Dmitry Rybolovlev, according to the Tribune de Geneve.

Bouvier and Rybolovlev have been locked in a contentious international legal battle for years. The billionaire alleges that Bouvier defrauded him by misrepresenting sale prices and taking outsize commissions in dozens of multimillion-dollar art deals. Rybolovlev has been doggedly pursuing Bouvier in a number of countries, with varying success.

According to French-language reports, Ueli Maurer, the head of Switzerland’s Federal Department of Finance, authorized a special investigation of Bouvier in March on suspicion that he had been avoiding taxes. The Swiss Federal Tax Administration (FTA) then began to conduct search and seizure operations. Authorities recently froze a real estate property in Geneva that Bouvier owns through a Swiss company to guarantee he would repay back taxes if he were found liable.

Bouvier also allegedly received dividends from two companies tied to him—one in Hong Kong and the other in the British Virgin Islands—that investigators reportedly suspect should have been subject to Swiss tax. Authorities are also looking into claims that Bouvier, who has had an address in Singapore since 2009, actually resides in Switzerland.

An attorney for Bouvier, Pierre-Alain Guillaume, told artnet News that his client voluntarily submitted to an audit by Swiss authorities in 2015, has actively cooperated with all investigations, and “does not owe a cent” to Geneva’s tax administration at this stage. He added that the figures tossed around in court were overblown and told the Tribune that the companies under scrutiny had no unreported profits.

The investigations stem, the lawyer claimed, from “a severe smear campaign” initiated by Rybolovlev’s team. From the beginning, he said, “a far-left politician based in Geneva alleged that Bouvier had committed tax offenses in Switzerland.”

The audit is expected to last three to four years, Guillaume said. “This is ‘only’ a tax issue,” he noted; no criminal investigation has been launched. “Yves Bouvier is confident that he will prevail,” he said.

Geneva mayor Rémy Pagani confirmed the investigation in an email to artnet News. In an interview with the Tribune, the mayor said he first encouraged authorities to look into Bouvier more than two years ago when he was a member of Parliament and raised questions about the CHF 2 billion ($2.1 billion) worth of art Bouvier had sold to Rybolovlev. A spokesperson for the FTA declined to comment.