Hah!Made an account just to suggest this.
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I haven't tried it yet, but what if you sold your resource to an AI, than destroyed the improved tile. Could you rebuild the tile and sell it again or would the trade be off?
If this worked I could see it being very useful when AI trades you lump sums.
Either ways this will be a hard to stop exploit :
1) If your trade agreement stays, you get the money for free.
2) If your trade agreement cancels, it will be a good way to cancel a trade, any trade you don't want. Just include a spare resource in the original agreement and destroy the resource tile when you want to stop. (Maybe even peace treaties?)
In all cases if you get gold for the resource, then spend the gold, i don't see how they can cancel the trade agreement since they can't take the money back.
The only way to fix this would be to make a resource tile improvement invulnerable to self sabotage / only make it destroyable by other civs.