The real estate agent Cyrus Greenspon has filed a petition in Manhattan Surrogate’s Court demanding compensation over a family-owned Ad Reinhardt painting, whose sale he says was handled suspiciously by his step-sister Amy, co-owner of the Algus Greenspon gallery in New York, DNAinfo reports.

According to Cyrus, last year Amy sold the 1953 Reinhardt  blue monochrome for just $375,000. Just a few months later, during the Swiss edition of Art Basel last summer, über-gallerist David Zwirner sold the very same painting for a whopping sum reported to be between $5 million and $10 million.

For Cyrus, the crux of the conflict lies in Amy’s professional relationship with Pace Gallery, where she worked for several years. Cyrus says that Amy got Marc Glimcher, president of the powerful Pace group, to write a letter giving a very low estimate of the painting.

Cyrus argues that both he and his brother had strongly opposed the sale, demanding that the painting—which originally belonged to their father, the late art collector William Stuart Greenspon—was sold at auction.

Cyrus is still struggling to understand how the painting, which Amy sold to her London-based friend and former Pace Gallery co-worker James Lindon, ended up in David Zwirner’s hands, but he also says that Amy and Zwirner know each other.

Greenspon’s petition claims that Sotheby’s had made a previous valuation of the work of $300,000, but it was only based on a photograph of the work. Another valuation made by the Internal Revenue Service had estimated the painting’s value at $900,000.

Bloomberg’s Art Basel 2014 sales report opening lines were about how a 1950s abstract blue painting by Ad Reinhardt sold so quickly at the booth of David Zwirner that even millionaire collector Steven A. Cohen missed his chance. Apparently, the painting sold barely an hour into the preview of the art fair.