The game's author, Jeffrey Allers, posted the full rules in English here. You'll want to take a look at that, because there's actually a twist to collecting the Joker piece, that I noticed in the German rules and asked him about via email.
If you decide to collect the Joker slice, another player may trade for it on a future turn.
Jeff provides a complete explanation and gives and example in the link. I can't wait to try this out!