Let's make this clear:
1) I finish gambling quest, beat Sendler and choose a bomb as a prize. No option to get the Troll head.
2) I finsh the Troll Trouble quest, I chose to take the non-violent path. Sendler told me I could play for the Troll head, but that option does not show up when I play him, even if I win. Okay, guess I don't really need it then.
3) I stumble upon a new quest that I somehow missed earlier called Melitele's Heart. Let's see, I already have all the other ingredients, only missing a Troll tongue.
Imagine my frustration that I am forced to kill the troll to complete that quest (despite me deciding to spare the Troll in an earlier quest, and that the guy who asked me to spare him was probably grateful for that). I decide to spare him, because when I'm trying to be a reasonably good guy, killing the troll after accepting the rewards for sparing the troll is kind of evil.
The thing is, you should be able to play for the troll head, even after beating Sendler as part of the gambling quest, and they should not be connected like that. What would you do in Geralt's place? You'd probably explain that you need that troll head for something that you didn't know earlier, or that you just need the tongue, and try to come to an agreement.