How to appeal an account deactivation for Counterfeit product claim

discussion.Hi

We purchased stock from a manufacturer 6 years ago. A buyer, having had the product for 3 months, has claimed it is counterfeit. We have the original invoice but Amazon require an invoice dated in the last 365 days and before the date of the claim. We are unable to provide this since the line has only been purchased once

How would you think we can resolve what seems to be something of a 'catch 22'