Dear friend,
I know that we planned to meet this afternoon, but unfortunately, I just got a phone call from my boss saying that he needs me right now for the company. I am sure that it is an emergency and I guess I will have to go now, so that he can get the help he needs. So I may not be able to keep my promise, and I am really sorry. But I'll probably be free tomorrow morning, what do you think? I hope you do not mind about my absense, and consider changing the plan to tomorrow morning.
XX
希望能帮到你~~ =)