Dear Tom,
I’m very sorry that I can’t go on a trip to Beijing with you next weekend, which I have promised you. I’m writing to tell you the reason for it.
Just now, my cousin, Li Ming, who went abroad for further education last year, informed me that he would return the next Saturday morning. He asked me to pick him up at the airport with his parents. As you know, we haven’t seen each other for about a year, so I’ll have to help him in the first place.
I really hope that you can accept my apologies and understand me. I would appreciate it if you allow me to fix another time to show you around some places of interest in Beijing.
Wish you have a good time this weekend!
Yours,
Li Ming