Dear Mr.Smith,
I'm very sorry to let you know I'll be unable to keep our appointment this Friday for our business talk as my manager asked me to go to Beijing to deal with an urgent matter and won't back until next Monday.
May I postpone our appointment to next week after I come back from the business trip.
My apologies for any inconvenience this may cause you,Mr.Smith.
I'll be looking forward to your early reply.
sincerely,