The players know they are coming up upon a fight with a dragon. They are approached to buy “Dragon Immunity Ring”. This could be a shady fellow in a back alley, a trusted merchant that has them in the back storeroom, or even a benevolent priest from the local temple.
They are told the the ring gives immunity to the dragon’s breath weapon type and resistance to bludgeoning, piercing or slashing damage from the dragon.
The ring does not work. It is a normal ring. It is still has the value for the precious metals and gemstones that make it up.
If the characters beat the dragon, they find in the dragon hoard a crate of rings identical to their dragon immunity rings.
The image above is from AlexFoxJewelryStudio on Etsy. https://www.etsy.com/listing/585862136/dragon-ring-silver-dragon-jewelry-celtic?ref=related-3