Have a Neko survive 1 round with no fish, next round feed it 2 fish and sell it.
You get 84 gold back, if you undo the fish and then sell you get the cost of the fish (60) + the sell value (42).
Having to remember to undo is dumb. You should be refunded the whole fish value as it was value added in the current wave.
Just hit this in a ranked game and losing 20 gold early in the game doomed me, not fun.