Tor Iver Wilhelmsen
I don’t think you can discard a previously played card. The act of discarding is specifically meant to disallow you from playing a card.
In this game, when discarding as a cost, it must be discarded from your hand.
Ah, yes, I was a bit drowsy when I read the section on discarding in the rulebook. It makes sense that a played card cannot work as the discard cost of something.