In a medium bowl mix the vinegar, pepper and cumin. Clean the liver and place in the marinade. Refrigerate for 25 minutes.
In a large skillet melt the butter and olive oil over medium heat. Add the liver to the skillet and cook for about 3 minutes on each side.
Add the onions, garlic, tomatoes and salt. Cook for 5 minutes more. Cover the skillet and cook for 3 minutes more or until cooked through.Sprinkle fresh cilantro on top and serve over white rice.