Look at it from a couple different perspectives.
If you were at the end of your life and looked back at going to the fundraiser, would you be fond of that memory? If you were at the end of your life and looked back and didn’t go, would that be a big regret?
Now look back and picture going to Berlin. Would you be fond of that memory? If you didn’t go, would that be a big regret?
I can appreciate that you value keeping your word, but at the end of the day you are a volunteer. Volunteering at the expense of becoming resentful doesn’t seem like the right thing. Perhaps I’m not clear on what your commitment was or something else in the coordinator’s perspective, but from what you describe it sounds like you are feeling guilt tripped.
In my experience, if I am too altruistic I become resentful and depleted; when I am too self-serving I get stagnant and lose perspective. Neither is inherently good or bad, but it is a matter of balance and motive which can determine if it is positive or not.
Good luck on making a choice that sits well with you.