The trial of Helge Achenbach continues to yield more details over the influential German art adviser’s dubious business practices. Achenbach stands accused of defrauding the late billionaire and Aldi supermarket heir Berthold Albrecht on 23 separate occasions resulting in estimated damages of €23 million.

On Monday, the testimony of Thomas Kellein, a renowned art historian and close associate of the accused, revealed more information on Achenbach’s alleged illicit dealings, FAZ reported.

In a two and a half hour testimony before the Essen District Court, Kellein alleged that Achenbach had charged the pharmaceutical entrepreneur Christian Boehringer “high price premiums” for some art sales. Kellein testified that as he worked with Achenbach in an advisory capacity, he initially wasn’t involved deeply enough in the adviser’s business arrangements to become suspicious. “I thought, well, that’s the way it is,” he said.

The transactions between Achenbach and Boehringer were handled via the now-dissolved art consulting firm Berenberg Art Advice, a subsidiary of the private Berenberg Bank. At the time, Achenbach and his co-defendant acted as managing directors of the art advisory division. Kellein was hired to help build the Boehringer collection.

Kellein noticed that “something wasn’t right” following three transactions between December 2012 and May 2013. Kellein said that “the penny dropped” during the sale of an early Baselitz work. Achenbach had charged premiums that he “couldn’t support anymore.”

Kellein informed the directors of the Berenberg Bank about his suspicions over the increased premiums. “I realized that it was about buying art cheap and selling expensively.” Kellein continued, “It should have been advice!” Shortly after Kellein came forward, the bank dissolved its art consulting division.

The €1.8 million dispute between Boehringer and Achenbach was later solved by Berenberg Art Consulting in an out-of-court settlement.