Below is the sample letter of informing customer of inability to execute order.
Dear Mrs. Brown,
We thank you for your order (No.12345) for 6 dozen factory work clothes.
We regret to inform you that we will not be able to complete the job in one month as you stipulated. This is because our workload at the end of the year is very heavy and we have many other orders to fill. We estimate that the earliest date on which we can deliver the work clothes will be the first week of March next year. If this is acceptable to you then we will take the job. Otherwise we recommend that you try another tailor.
We apologize for causing any inconvenience.