I have had the same thing happen where I had expertise in an area the therapist asked me about. But I did not pay the therapist for that appointment or the next one. And then we went back to usual.

But as to your other question about what could you do?
I do believe it is perfectly okay to tell the therapist no, it is your time. I think it is okay to tell a therapist if he wants info about something from you that you are the expert in, he can arrange an appointment with you for it at your normal hourly fee.