German term (edited): Wertersatz f�r eine Verschlechterung leisten

This is how I would put it:

"The customer must compensate for any deterioration (in value) following use of//due to the use of the matter//object in accordance with the instructions."


Compare link below, from where I took the following quotation:
"4. Return policy
The customer is able to return the goods free of charge and without giving reason within 14 days of receipt as long as the goods are returned by the due date. On orders of up to 40 Euro, the customer must carry the return costs unless the delivered goods are not in compliance with the order. Unstamped packages will not be accepted.
The customer is required to compensate for any deterioration following use in accordance with the instructions. This does not apply when the deterioration only occurs on inspection of the goods. This is in regard to all tea products. Accessories will only be taken back unopened in the original packing and unused.
The customer must inform Venalicia immediately of any obvious defects or transport damage."

I hope this helps.
Regards,
Susanne