On Wall Street, Andrew J. Hall is known, even feted, as an aggressive trader with nerves of steel, capable of earning a $100 million bonus in a single year. His particular skill: spotting opportunities in high-stakes oil markets.

When he turned to constructing a personal art collection, he did so with similar ambition and shrewdness, amassing an impressive trove of 5,000 works by hundreds of artists he loved, including Leon Golub, an American postwar painter of expressionistic, heroic-scale figures.

But when Mr. Hall set out last year to stage an exhibition of Golub’s art at the private rural museum he operates in this small village, he found out to his surprise that more than a third of the Golubs he had bought were forgeries, according to a lawsuit he filed last month.