The first thing that came to mind was the story about R. Shimon ben Shetach whose pupils purchased a donkey that had a priceless jewel hanging around it’s neck. R. Shimon ordered them to return the jewel since he only purchased the donkey.
However, the more I thought about it, I realized that this case probably isn’t relevant since in R. Shimon’s case, you could make the argument that he purchased only the donkey and not the jewel, which is external and separate from the donkey. Here, however, the negatives themselves were the item sold — it’s not like the negatives were external to something else that was purchased.
Sadly, I’m no talmid chacham, so my ramblings on this are probably meaningless and k’neged halacha anyway. 🙁
The Wolf