Responding to different requests to contribute a book chapter I received a request to contribute a book chapter about a month ago, which I turned down. This was from a university in another country, and from someone I have personally met.

Today, my own supervisor asked me to contribute a similar chapter to a book which she has been asked to be an editor of.

Given that the two requests happened in a relatively short time, and the reason I gave for declining the former request might still stand, would there be a problem if I accept the latter request, when I have declined the former?
This is not an ethical issue at all. You are free to contribute to whatever book / project you like. There is no obligation that you can't join a different book project just because you turned somebody else down before.