Dear Khun .....(Customer),
Apologize for short notice but (name of boss) will have to attend an important meeting on 26 august 2015
May I propose to reschedule ourmeeting (ชื่อ/เรื่อง meeting) to other day. New date andtime will be advise shortly.
Sorry for any inconvenience this may cause. Please do not hesitate to contact me for any assistant. I am always at your service.
Best regards,