When a Sotheby’s official appraised a painting by Pieter Bruegel the Younger in 2005, he set its value at $500,000. But when the owner of the 17th-century work, “St. George’s Kermis With the Dance Around the Maypole,” sold it at a Sotheby’s auction in 2009, it drew more than four times that amount, or $2.1 million.

What explains the wide gap between the estimated and actual value of the work?

The expert from Sotheby’s would later say that, after the appraisal, there had been a spike in prices created by a “large influx of Russian buyers” eager to obtain old masters and that the painting had been cleaned before being sold.

But the United States Tax Court took issue with that account in a recent decision.

In February, Judge Joseph H. Gale ruled instead that the expert had most likely placed a “lowball” estimate on that painting and a second work so as to “curry favor” with the owner, an estate facing a potentially large tax bill, and thus win the business of selling the works at auction.

His ruling came in a case where the estate challenged a determination by the Internal Revenue Service that the paintings had been valued too low.