Can you tell me How to say "no" to my boss

I arrived in Haining at 11:00 AM today.
I have to go to Haining every week . Just meeting a client. he is a good guy. I like chatting with him.
He said that he would to join in an exhibition about his company product in Shanghai At 24 August.
I replied him. I live in Shanghai. it is very near from my apartment to the exhibition site. I can help him if he need somebody to help him.
He smiled and told me. he would call me if he need.
Just then. My cellphone rang. It was my boss.
My boss siad"Gary. you must stay in shanghai at 24.August. I will have an exhibition in shanghai. and you must come and help me".
I felt very sorry for my client. but I had no idea how to refuse my boss.
can you tell me the way how to refuse my boss.
thanks a lot

Can you tell me How to say "no" to my boss?
I arrived in Haining at 11:00 AM today.
I have to go to haining every week just to meet a client. He is a good guy, and I like chatting with him.
He said that he will be joining an exhibition of his company's product in Shanghai on the 24th of August.
I replied him that I live in Shanghai and the exhibition site is very close to my apartment. I said that I can help him if he needs anything.
He smiled and told me he would call me if he needed any help.
Just then my cellphone rang. It was my boss.
My boss said "Gary, you must stay in shanghai until the 24th of August. I will have an exhibition in Shanghai and you must come and help me".

I felt very sorry for my client, but I have no idea how to refuse my boss.

Can you tell me how I can refuse my boss?

thanks a lot