What is your first response and how do you handle it when a customer forgets his/her transaction and he/she said ''i don't do this''?

First tell him the date & time that he/she initiated the transaction and try to persuade him. If he does not agree to that then provide him with a proof like, if the transaction was over phone, provide him with a list of phone calls that were attended that day and time or any written proof like any documentations that were filled or signed by the customer.