Dear

After reading your letter of (date) , I can thoroughly
understand why you are running out of patience.

While it would be easy to place the blame on our computer,
this poor fellow has received enough abuse since joining
our firm. After all, he only follows the orders that are
given to him. Therefore, please accept my apology for the
delay in refunding your money for (basis of adjustment)

Our bookkeeping department has been instructed to issue a
check to you at once, which you should be receiving within
a few days.

I am grateful that your letter was brought to my attention
and I appreciate your perseverance in settling this matter.
Once again, I am very sorry for the inconvenience this has
caused you.