We are in receipt of your letter dated ……………… and learned the cause of your running out of patience.
We are indeed sorry for the delay in refunding your security money; it actually happened due to a mistake in feeding the computer. The accounts department has been instructed to issue a cheque to you at once. You will get the same within three days by speed-post.
I am grateful to you as your letter has brought the mistake to my attention and I appreciate your perseverance in settling this matter. I personally apologize for the convenience caused to you.