~Snip
That's why it's called gambling in the first place, nothing is certain so don't be surprised if actually Dortmund do the unreasonable and came out on top of Real Madrid come that final day of play, so I would just tell you some advice which is to gamble on the odd with little funds that won't affect you even if Dortmund doesn't win because the whole world of football expect Real Madrid to win even the bookmakers but sometimes things do change so goodluck.
Even though Dortmund are not favorites in the final match, it doesn't feel like they really deserve to be underestimated. Dortmund have proven themselves to reach the final by beating many strong teams since the group stage, so I have no reason why they should be underestimated by some fans.
It's reasonable to expect Real Madrid to win the Champions League title given their experience and finals record, but it's not at all a good idea to say Dortmund don't have a chance. Even the bookies cannot completely write off the odds for Dortmund to win the match, which means there are odds being considered for their victory.