参考答案：

Dear Mrs. Benton,

I’m so sorry to have to write you this letter! After accepting your kind invitation for dinner on Tuesday, May the fifth, I now find that I cannot be there after all.

I have just been called to Shanghai on business. Although I tried hard to put off the trip until after May the fifth, I was unable to do so. The Office Head told me that the business tour was quite urgent. A company in Shanghai threatened to discontinue buying the products we make. I was required to negotiate with it about the matter.

I had been looking forward with great pleasure to seeing you, and I now feel very bad about this unexpected development. I hope that you understand me and that we’ll meet in the near future.

Sincerely yours,

Li Ming

解析：

Notes: after all 毕竟，终究。on business 出差。put off 推迟。discontinue 中断。feel very bad about 感到抱歉。