Helping in battle

[evikour]

If a player joins a battle as a helper is that irreversible or he can back out if the terms of the battle/agreement change?
Example: Player A draws the Ninja Giant and can't beat it on his own, so he asks for my help and offers me 2 treasures (out of 5). I accept and we are about to win when Player B throws the Big Honking Sword of Character Whupping and enhances the monster by 10. We are now losing and Player A says that he will sacrifice his "precious" Critical Hit card to win the battle but wants both the additional treasures the Sword card grants if winning the monster.I disagree because he still needs my help even with the Critical Hit played and demand to take one of the two treasures. He doesn't accept and I don't want to take part in that battle any more.
Can I leave the battle?What is the legal thing to do?Do I have to take the deal Player A offers no matter whether I like it or not?
(I had an enhancer myself to sabotage the battle at that point but since the monster's Bad Stuff was death I chose not to play it)