A second-hand dealer has won a case to retain €4.2m ($4.6m) from selling a rare African mask found in an elderly French couple’s attic. His assistance was sought by the couple who asked him to clear the attic. The couple sold the mask to him for €150 (£129; $165).

The couple claimed they were misled about its value and sued after selling it to the dealer for €150, as reported by the BBC. But, the judge ruled that the couple had underestimated the true value of the Ngil mask.

The artifact is a rare piece crafted by the Fang people of Gabon, with only around 10 known in existence. This artifact, worn by members of the Ngil secret society, was historically used to identify and confront troublemakers, including suspected sorcerers in villages.

The 19th-century wooden mask, acquired around 1917 by René-Victor Edward Maurice Fournier, a French colonial governor, and the plaintiff’s grandfather, was in the family’s possession until sold to a dealer. The mask was later auctioned to an unknown buyer, which prompted the couple to seek a share of the proceeds, alleging the dealer misled them about its true value.

The dealer denied knowledge of the mask’s true value and asserted that he showed goodwill by offering the couple €300,000, the initial valuation. His lawyer contended that the couple neglected the primary responsibility to research the item’s actual worth before the sale.

“When you’ve got such an item at home, you should be a bit more curious before giving it up,” Patricia Pijot told French media. The judge ruled in favor of the dealer, citing the couple’s failure to conduct due diligence on the “historical and artistic” value of the mask.

The initial offer of €300,000 was retracted after the couple initiated legal action. Frédéric Mansat Jaffré, lawyer for the couple, said: “The judge has created a precedent… You or I will now need to ask a professional before going to see another professional.”

Gabon’s request to halt the mask’s sale, claiming rightful ownership, was also denied by the court. The West African country was a French colony at the time Fournier acquired the mask. Many African art pieces, including tens of thousands from the continent, are held abroad due to colonial-era removals.

French President Emmanuel Macron recently advocated for the restitution of African art. “I cannot accept that a large part of the cultural heritage of several African countries is in France,” he said in 2017.