* McCourt promises to return club to former glories * Deal has cost 40 million euros - media report (Adds details, changes slug) By John Irish PARIS, Aug 29 - Frank McCourt, former owner of the Los Angeles Dodgers baseball team, said on Monday he had entered exclusive negotiations to buy Olympique de Marseille and vowed to restore the club to their former glories. Billionaire businesswoman Margarita Louis-Dreyfus, the Russian-born chairwoman of Louis Dreyfus Commodities trading house, said in April she was looking for a buyer for the Ligue 1 side. "Margarita and I just executed a contract for us to buy OM, the greatest football club in the French league and one of the best in the world," McCourt told a news conference alongside Louis-Dreyfus and Marseille mayor Jean-Claude Gaudin.