How do I repair giving away toys my son thought I was only storing?

The charity collector had already left when my eight-year-old asked where the green dragon was.

I had said we needed to clear the bedroom floor and that toys in the blue box would go away. I meant donated. He thought I meant put in the cupboard, because that's where we store the toy train between visits from his cousin. I didn't open the box with him before the collection.

The dragon hadn't been played with for months. That was my reason for including it, but he says he had been keeping it for a comic he wants to make. I found the first page under his bed after we'd argued.

I called the charity. They will check whether the box is still available, but can't promise. Buying the same dragon secondhand is possible, although he says another one would not be his one.

I needed the room usable. I also chose an efficient way to clear it that skipped the conversation. What can I do if the toy can't be recovered, and how do we make the next clear-out something more than me deciding what still matters to him?