I think you did the right thing

By telling him that you are flexible with the salary as long as it is a good fit for you. Just leave it this way. You don't have to bring it up again in the thank you note. Is he the hiring manager or recruiter? If he is the recruiter, he does not have much saying in the final offer on salary anyway. Don't sell for short. If he is the hiring manager, not very professional for him to ask minimum salary requirement or negotiate. He should leave it to HR.