Heat the butter in a heavy skillet over medium-high heat. When the butter is melted ad begins to bubble, add the sausage. Cook the sausage for a couple of minutes, or until it begins to brown. Add the onion and garlic, and cook for another minute, until they are translucent but not browned. Add the curry, stirring and cooking to bring out its flavor. Stir n the water, then the ketchup. Bring the liquid to a boil, then lower it to a slow simmer. Cook the sauce, with the sausage n t, for a few minutes, or until the sauce thickens slightly.