How do you respond in a negotiation with a customer when the customer’s demand is unreasonable?

Politely advise the customer what you are capable of delivering and let him/her know that you do nit promise what you cannot deliver

You need to advice the customer what you have.

Be truthful with the customer & give an example that we made the best choices we could in some cases but in this case this order is not useful for you as an advice if he still insists offer him another choice similar to his requst .