Nicolas Puech is an heir to the Hermès fortune whose riches are shrouded in mystery. He either is or is not a billionaire, and may or may not have a sizable stake in his family’s luxury brand, depending on when and where these matters come up.

A new lawsuit filed in federal court late last month says that Mr. Puech recently claimed he does hold that stake, about 5 percent of the company, and had signed a deal to sell more than six million shares in Hermès to the royal family of Qatar. But Mr. Puech has also previously told courts in Switzerland, where he lives, that his shares had disappeared in the hands of a wealth manager.

The complaint in federal court in the District of Columbia, initially public but now under seal, accused Mr. Puech of failing to honor the sale, adding fresh intrigue to the enduring enigma of his wealth and offering a glimpse into the luxury ambitions of Qatar’s monarchy. The original suit was rejected on a technicality by the court, and the plaintiff has refiled with a motion to keep it under seal.

Mr. Puech, 82, is a great-grandson of Thierry Hermès, a 19th-century saddle maker who turned his business into a fashion powerhouse revered even by other fashion brands. Hermès — known, among other things, for the exclusive Birkin bags it sells only to insiders — was valued at $300 billion in mid-February, just days after Mr. Puech signed a deal to sell his shares, then worth over $15 billion, according to filings in the suit.

It is not the first time Mr. Puech and his slice of the family fortune have been the subjects of great debate and litigation.

In 2023, he made waves after moving to adopt his middle-aged, married Moroccan gardener to bequeath him half his fortune, prompting resistance from a charity he had formed, which expected the inheritance.