Peter Grosz, the son of German Expressionist George Grosz, recently passed away, and the obituaries mentioned that in recent years he "became involved in a long lawsuit against an art dealer. He claimed that Serge Sabarsky, who sold George Grosz's work for his sons for years, cheated them by buying it himself under the table without getting a competitive price." The suit was apparently settled a couple of months ago. A New York Times article ($) from 2001 (by which time the case had already been going on for six years) has further details on the dispute:
"In th[e] suit, filed in State Supreme Court in Manhattan, the Grosz estate claims that Mr. Sabarsky secretly acquired 440 Grosz works for himself, primarily drawings and watercolors produced in Germany in the 1910's and 20's, the period of his most valuable artwork. Another 110 works are missing, the Grosz estate says. ... 'What he did over the years was to buy art saying he sold it to third persons, but buying it for himself and giving us a fictional sales price,'' said Peter M. Grosz, 75, elder son and executor of his father's estate. 'We got paid for it, but it wasn't on an open market, and it wasn't done competitively. It's up to an art dealer to increase the price, but because he was buying it for himself, he wanted to keep the prices as low as possible.'
...
"Mr. Sabarsky's deal with the Grosz family was made with a handshake: Mr. Sabarsky would promote the works and receive a 33 percent commission for retail sales, 40 percent for wholesale. For several years Mr. Sabarsky made an annual accounting to the brothers, but from 1983 to 1990 there was just one. That accounting and others, Peter Grosz said, referring to papers the Grosz family said were from Mr. Sabarsky, did not list specific clients, just a notation about whether a sale had been wholesale or retail. ...
"In 1994 the Grosz estate fired Mr. Sabarsky. The next year, having come to believe that Mr. Sabarsky had been secretly acquiring art for himself, the Grosz estate brought suit. ...
"The original suit, filed in 1995, seeks $6 million in damages."
Sabarsky was the co-founder (with Ronald Lauder) of the Neue Galerie in New York, recently making big news for its $135 million purchase of the Klimt Adele.