أنشئ حسابًا أو سجّل الدخول للانضمام إلى مجتمعك المهني.
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.