Do you spend large amounts often in each window? If so, 400M isn't that much since they are constantly shifting funds to balance the books.
Not sure if it's relevant to FM but Parc de Princes is in the heart of Paris so the cost of the land (to buy the stadium) would be extortionate and you don't have a stadium to sell to make up most of the funds to build a new ground.
My advice would be to groom youngsters witht he intention of sellign them to bring in healthy profits from Transfer Windows.
Attacking football and winning matches pulls crowds so try to keep the fans happy and sell out the stadium for every home game.
Convince the Board that the fanbase is there waiting to move in.