Maybe it could help you both?
Here's an alternative point of view. Use the experience to create a relationship with more collectors. Try promote it that a website, or author, or product chose to use your painting.
When you advertise new work also mention that you are honored that someone is using your work and chose to you use your art.
If you write to them you may even be able to convince them and to give you a shout out. What about sharing your work and asking them to help you too both for free? I do this all the time with authors and magazines and never charge them. The "good will" helps me sell my art.