A card is not played until the cost is payed, so what would happen if my opponent:

1. Takes a card from his hand, and discards plays it, without paying the cost
2. Re-rolls a dice
3. Finishes his turn

In a casual even we probably just pay the cost and play on in a competitive event what would happen?

OP edited

In a competitive environment, a judge would be called and it would be up to their discretion, the player would likely either be forced to retroactively pay the cost and be given a warning, forced to rewind to the start of his turn and do it over properly, or given an automatic loss. Depending on the history of the player, the exact board state (how impactful the mistake was), and any previous issues with that player.

I'm assuming the re-roll came as an effect of the card that was played. If they not only played a card without paying, but also rerolled without having the action to do so or discarding a card, then that is a series of violations in a row and really doesn't look good on them as a player. It would still be up to the judge's discretion, but it may not end well for them.

So your opponent plays a card without paying its cost? At that point I'd ask them to pay the cost or return the card to their hand.

Then they re-roll a dice? At that point I'd ask them to return the dice to it's original side, and discard a card before re-rolling.

Unless there's something I'm seriously missing from the question here this seems to be a very open and shut case of someone misplaying. I suppose in a competitive environment you could call a TO and look to have them disqualified or forced to take a loss on that particular game, but I highly doubt someone in a competitive environment would misplay in this fashion to begin with. Mistakes do occasionally get made, but I don't think I've ever seen someone flat out just play a card for free then re-roll a dice...

As stated unless you catch it and call a judge. The judge will instruct the player to pay it, or rewind the play or its a dead card (placed in discard) and it ends the turn. If you can recall what the dice were before the reroll all dice would be put back to that side and damaged removed it it did any to you.

I'm unclear as to what's happening. Is he trying to both play a card AND use it as a discard to re-roll? Because that's what it sounds like.

When you say he plays the card, do you mean he also resolves it? because you don't resolve a card until/unless you are able to pay the cost.

In any case, there's either a case of cheating happening, or a serious misunderstanding about how to play on a basic level. If I'm playing in a event, or against an experienced player, I ask them to stop and explain what they are doing. Depending on what they say, if it's an event I call a judge over to rule how to deal with it. If it's just a casual game, we back up and they do something legal with their turn.

It sounds like he discarded to reroll dice.

Okay, he can discard a card to reroll dice, that is fine.

He can pay the cost to play the card and resolve its effect and will not reroll dice unless that effect instructs dice to be rerolled.

He cannot do both. He can discard a card to reroll dice even if that card's effect would be to reroll dice at a cost. Whatever benefit that card effect has in addition to rerolling dice will not come into play however if he discarded to reroll.