Authorities in Geneva seized Amedeo Modigliani's Seated man with a cane, which is worth an estimated 22 million euros, in April, following the leaking of millions of documents from Panamanian law firm Mossack Fonseca.
French farmer Philippe Maestracci has filed several suits against art dealers Helly Nahmad and David Nahmad, claiming that the picture was seized by the Nazis and rightfully belongs to him.
He says it belonged to his grandfather, British Jewish antiques dealer Oscar Stettiner, who fled Paris ahead of the German occupation.
Mondex Corp, a Canadian group that helps find looted assets, has appealed to the US authorities to help Maestracci gain possession of the work.
The Nahmads told a New York court in 2011 that they were not the painting's owners and were exhibiting it for a company called the International Art Center.
The Panama Papers leak revealed that David Nahmad was the company's sole owner.
Proof of looting disputed
France's Le Monde newspaper last month claimed it had established that the picture was seized from Stettiner in 1944.
But last month the Swiss authorities said that the painting could be returned to the Nahmad family.
Nahmad, who is Jewish, this weekend insisted that there was no evidence that the 1918 painting it belonged to Stettiner and told the AFP news agency that he would never take art looted by the Nazis.
Documents provided by Nahmad's associates to AFP show a Modigliani self-portrait was one of three works mentioned in a 1947 legal claim.
But Seated man with a cane is not a self-portrait.
According to Le Monde it is a portrait of chocolate manufacturer Georges Menier and was exhibited at the 1930 Venice Biennale, after Modigliani's death, along with a portrait of Menier's wife, Simone.