Pat the chicken dry and dredge it in the flour, shaking off the excess. In a large skillet heat the oil over moderately high heat until it is hot but not smoking and in it brown the chicken. Transfer the chicken with tongs to a plate, add the shallot and the grapes to the skillet, and cook them over moderately low heat, stirring occasionally, for 3 minutes. Add the wine and the brandy and simmer the mixture until almost all the liquid is evaporated. Add the broth and the brown sugar, whisking, and boil the mixture until the liquid is reduced by half. Add the chicken and any juices that have accumulated on the plate, simmer the mixture for 2 minutes, or until the chicken is just heated through, and transfer the chicken with tongs to 2 heated plates. Remove the skillet from the heat, whisk in the mustard and salt and pepper to taste, whisking until the sauce is combined well, and pour the sauce over the chicken.