Answer:
- Exrpress understanding and explain why his/her demand is not acceptable.
Explanation:
When dealing with a customer, and you consider he/she is not rigth, you should be able to express your point of view in a respectful way but clearly  preserving your rights. That is assertiveness.
You must confront him/her in a constructively way: make it clear why the claim is not correct or fair, even how it affects the your or the company's right: you undersant him/her but he/she must understand you too.
You should prevent the situation from escalating to greater proportions but you should not give in to unfair demands that involve a loss for the company.
