Hmmm, while avoiding you seems unusual on my part, isn't it like you judged him way too early just because of that one single action coming from him?
A few days to weeks after avoiding you, what's the update now? Is he avoiding you still?

Observe his movements and if possible, follow his gambling journey, but don't get attracted to it yet again. From there, make a decision, and as for the money that he promised? Why just not tell him directly that you don't want that money anymore. Come to think of it. Your friendship has been affected just because of that 10% winning that he promised. Do you think that it's worth it? I don't think so that's why just approach him and tell him that you will not get that money anymore. You're his friend so he will understand you that's for sure.

I believe that both of you are matured enough to handle this kind of situation. Approach him and directly tell him that you will not get that money anymore. Maybe give him some advice as well that would help him since you have way more experience compared to him already.