Giving a talk on a different topic than what we discussed I'm a postdoc in mathematics. I had a conversation with a professor after he gave a talk at my university, and he seemed interested in my ideas. I sent him an email the next day to elaborate on our discussion, and he replied by inviting me to give a talk at his university. I accepted the invitation.

However, I don't feel confident about giving a talk on the subject of our conversation. I wrote something about it in my thesis which I sent to him in my email, but it was just some undeveloped ideas (not central to the thesis), and I am certainly not an expert in that area. Is it okay to give a talk on a different topic? 

He didn't specify what he wanted me to talk about, but we didn't discuss anything else than that, and it seems like our only point in common. So, I fear it would be awkward to give a talk on my true expertise area. Or is it?
If you haven't come to an agreement on what you would discuss, then I think you are pretty free. But as a courtesy, if nothing else, send him a note about your topic. If he disagrees you might want to re-think it. 

However, a talk about partially unformed ideas, if presented that way can be very valuable. It can give ideas about research the listeners might explore. If you do it right, it can give ideas about how to come up with such ideas and how to begin to tackle them. What insight do you have into the problems? 

In the best case it can lead to collaborations on that topic and others that might emerge from the exploration. 

In fact, a talk on "things worth exploring" is probably more interesting to a mathematician than one on "things done and finished."