Dear :

Thank you for inviting me to talk about the latest development in the production of healthy food at next month's Seminar. I would like very much to join the discussion and offer my own views to business people and professionals.

Unfortunately, I've got to go to [place] next month to visit some of our factories. I'm afraid I won't be able to attend the seminar.

If you plan to have other seminars in the future that will feature the same subject, I would be glad to speak, schedule permitting. Thanks again for the invitation.

Sincerely yours,

[name]

[title]