Dear Mr. Sation
With reference to your letter dated 3rd May 20xx, I am writing to apologies for the mistake we made.
The mistake was made because our computer broke down on 29 April. I can assure you we will send the cheque of $200 to you in two days. We will offer you 20% discount for you next stay.
Please accept our apologies for the inconvenience this error has caused you and we look forward to meeting you again.
Your sincerely
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