Dear greenshade:

I would not go on the twenty day travel because of that course you paid so much for and because you notified your boss about it ahead of time (the Saturday volunteer commitment is important but less important than the course because you are planning to resign anyway.

Because your plan to resign, and your concerns, I would make the confrontation about the travel as soft as possible. First, I would thank the boss for his/ her support in the past abundantly. Second, I would suggest (if practical) to do the travel on the days when your course is not taking place. Third, I would explain to him/her how important this course, how much you paid, in a way that may elicit her empathy.

anita