Dear Bob,
I am writing to make an apology to you for I forgot to return a music CD which I borrowed from you last week. Yesterday I have come back from Canada and I found it in my luggage. I am so sorry that you cannot listen to it at present.
The CD which I borrowed from you is made in Canada, so I believe you can find another one in your local store. How about your buying another CD by yourself and I will pay for it? If you cannot find the similar one, I would send it back to you through a post office. And if there are any other solutions you like, please let me know.
I am sorry again for my carelessness. Looking forward to your reply.

Yours sincerely,
Li Ming