Scripts

green belly, the game doesn't know what a hero name is, to which it should add +1 attack. Here's a working version + I added a function for a negative response (because in your case, as I see, there should also be a function for a negative response):

function arena1F(hero)
  arena1Fhero = hero
  gold = GetPlayerResource(1, 6)
  if gold >= 2000 then
    QuestionBox("/Maps/Multiplayer/arena/napadenie1.txt", "yes", "no")
  else
    MessageBox("/Maps/Multiplayer/arena/napadenie1nexvatka.txt")
  end
end

function yes()
  SetPlayerResource(1, 6, gold - 2000)
  ChangeHeroStat(arena1Fhero, STAT_ATTACK, 1)
end

function no() -- this is just an example of what will be called if you press No. The code can be changed to suit your needs, or the function call can be completely removed when refusing, by replacing "no" with nil in QuestionBox
  SetPlayerResource(1, 6, gold - 2000)
  ChangeHeroStat(arena1Fhero, STAT_LUCK, 1)
end

Trigger(OBJECT_TOUCH_TRIGGER, "arena1", "arena1F")