NEW YORK, NY.- Dmitry Rybolovlev survived the turmoil of the collapse of the Soviet Union, founded a bank in Russia in the 1990s and built a fortune of roughly $7 billion from the sale of a potash fertilizer company.

He managed to get that wealth out of Russia, acquired a share in a bank in Cyprus, a Monaco soccer team, a Greek island, and homes for his family in Switzerland, Monaco, New York and Hawaii.

But under cross-examination in a Manhattan courtroom on Friday, Rybolovlev acknowledged that the diligence he had applied in building his business empire was not always present when he and his staff bought $2 billion in fine art through a Swiss dealer.

Asked by a lawyer for Sotheby’s whether he thought the aide he had trusted to handle those high-priced acquisitions had done a good job, he told the courtroom, “Let’s put it this way: There was room for improvement.”

Rybolovlev has accused the Swiss dealer, Yves Bouvier, of tricking him into overpaying for the art he bought over a period of 12 years as he constructed a collection of masterpieces, including Picassos, Van Goghs and works by other artists.

Rybolovlev has said that he believed Bouvier, who is not a defendant in this case, was acting as a commissioned consultant, assisting him in obtaining rare artworks at a good price by negotiating on his behalf. But he later discovered that Bouvier was operating as an independent dealer, not solely as an adviser.

Bouvier had actually bought the artworks himself, for about $1 billion, before flipping them to Rybolovlev and keeping a $1 billion profit.

Bouvier has long insisted he did nothing wrong: that he had been clear that he was a dealer and was free to charge what he wanted for a work. After years and years of legal fights, the two men agreed to a confidential settlement last year. But Rybolovlev is still pursuing Sotheby’s, which he says in his lawsuit was aware of, and abetted, the alleged fraud.

Lawyers for Rybolovlev have said Bouvier pretended to haggle with phantom sellers in an effort to drive up the prices paid by their client. In court papers, they have accused Samuel Valette, a Sotheby’s specialist in the sale of impressionist and modern art, of helping Bouvier by providing inflated appraisals that were sent to Rybolovlev and by leaving Bouvier’s name out of transaction histories.

Sotheby’s was involved in a dozen of the sales, but only four are the subject of the lawsuit. The auction house has said that it knew nothing of any scheme and, as its lawyers argued again Friday, that Rybolovlev should blame only himself for failing to protect himself against price inflations.

In particular, said Marcus Asner, a lawyer for Sotheby’s, Rybolovlev’s aide had never formalized the agency agreement with Bouvier; had never asked to see contracts with the sellers from whom Bouvier said he was buying art; and had never checked to see where all the money Rybolovlev paid was going.

Although Rybolovlev’s aide was the point person, Asner said, the billionaire himself also never checked these things and ultimately was responsible because he was in charge.

“You understand the term ‘trust and verify’?,” Asner said.

“I understand,” Rybolovlev replied.

“Mr. Rybolovlev, are you familiar with the term ‘The buck stops here’?,” Asner asked later. “You’re the boss.”

One of the most compelling moments in Rybolovlev’s second day on the stand was his account of how he first became aware that he had been significantly overpaying for his art. Rybolovlev, speaking softly in Russian through a translator who sat by his side, described for the jury a chance meeting in St. Barts in December 2014 with a New York art adviser.

Over lunch with his girlfriend, a friend who was a collector and the friend’s wife in a restaurant at the Eden Rock hotel, Rybolovlev said, he struck up a conversation with art adviser Sandy Heller. Heller, Rybolovlev said, mentioned that a client — hedge fund manager and New York Mets owner Steven Cohen — had recently sold a Modigliani painting for $93.5 million.

It was, in fact, the same Modigliani that Rybolovlev had just bought, but Rybolovlev had paid Bouvier $118 million. He had not realized until that moment, he said, that Bouvier had been buying up the art and reselling it to him at higher prices.

“I was in a state of shock” after the revelation, he said. “My friend thought I was having a heart attack because I turned completely pale.” He left the table.

Rybolovlev also testified that he came to view the scheme as a betrayal by a man who had become his friend and whom he had admitted into his inner circle and had even invited to join his small birthday celebration in Hawaii.

At one point, he asked for a moment to compose himself as he recalled conversations with Bouvier.

“He behaved in a way that made you trust him,” he said. “There is a point in time when you start to completely and utterly trust in some person.”

Rybolovlev insisted that putting what he believed was their arrangement formally in writing or seeking contracts from sellers would not have stopped Bouvier because, he said, Bouvier would have produced another document from a purported seller and it would have been difficult to check its veracity.

Rybolovlev said his previous business experience did not help him in the art world because it was an industry that lacked transparency. He said he considered himself a victim of that lack of transparency. “It is important for the art market to be more transparent,” he said.

But Asner said that Rybolovlev himself had embraced the tactical stratagems of the art market when it suited him. He went on to describe a scheme concocted by Bouvier to cancel a meeting with the owners of one work they were seeking to buy, a painting by Leonardo da Vinci, in hopes that their feigned disinterest would lead the sellers to lower their price.

“Is it OK for you not to be transparent in a dealing with the da Vinci sellers?,” Asner asked. “Did you tell Mr. Bouvier when he proposed this ruse to not lie?”

Rybolovlev said he had decided to let Bouvier handle the negotiations.

“How he was going to do it, what he was going to say, that was up to him,” he said.

This article originally appeared in The New York Times.