How to turn down a MSc supervisor after we had already agreed to work together? I applied to a university in Canada for a MSc. As part of their requirements, I needed to have a supervisor in order to be granted admission to the program. So I did contact a supervisor and he took me in as his student with the condition that I brought my own funding. I said yes. However, the university has not yet offered me admission, I am still waiting for their decision. 

This was not my only option of course and last week I got accepted to a MSc/PhD program in a Max Planck university in Germany. Because it is much better ranked and students are given a monthly stipend, I took the offer because I thought it was a better option for me. 

How do I reject now the professor who accepted me on Canada? Should I tell him the truth? Is this a very bad move on my side?
Skip the "better-ranked" part (don't be petty); say you couldn't hold up your end of the bargain - to find your own funding -, and Max Planck offered a fully-funded position. I don't foresee any issues or grievances after a short, to the point mail to this sort.