Having been on the other side of a similar situation I can only echo jacquesm's comments. You made the choice to take on this fundraising obligation. You didn't ask this man first to see if he would commit to putting in some amount of money. You can't unilaterally obligate him to do so.

In my own situation, which did involve TNT, I found myself rather skeptical, if not cynical, about the Big Cancer Research Establishment. I think it exists primarily as a self-preserving bureaucracy and does precious little actual research. (I could be wrong; I haven't really looked into it.) Under the circumstances, I nonetheless gave an amount which I thought was generous (and which was larger than $10). It was still received as inadequate. This did not dispose me to be generous with similar requests in the future.