A gallery buys a body of your work. Several years later there are a few pieces which remain unsold. The gallery decides they want new fresh art or in my case, paintings. However, during this time your work has increased in price, so your paintings are now worth more. Do you do a direct swap or do you ask for the difference in price between what you sold them the original work for, and the new work? I tried the first approach and the gallery went ballistic. What's your view?