Preheat to medium high. Slightly oil grate to prevent fish from sticking. Season mahi mahi with salt and pepper. Combine lime juice, soy sauce, ginger paste and oil mix well. Place mahi mahi in plastic bag and add marinade. Marinade for 20-30 minutes. Grill for 6 - 8 minutes on each side.
Meanwhile drain marinade into sauce pan. Bring to a boil and reduce until reduced by half. Add brown sugar and butter.