Your customer has received an invoice, which lists several charges that need to be paid. However, the customer is disputing one of the charges. The customer was promised a price reduction on one of the items by the original salesperson, who has since left the company. The invoice does not show the reduced price that was promised to the customer. There is no record of a promise being made to the customer, and the old salesperson was not authorised to give the price reduction, which is against company policy. The customer is now very angry and is threatening to move his business somewhere else. How would you handle this difficult situation?