What will you do if during negotiation with a customer if the customer is demanding too much?

agreed with all

Well it's the skill, 

Be patient take & absorb pressure and convince the customer that you have the best deal in the town :)  

I will always affirm my and my organizations interest in this customer,i will keep assuring and confirming to him that as an organization we are keen in establishing such relationship but we have lines that we cannot cross in order to keep all customers and the organization healthy and be in an  acceptable growth stage within the market.

if he keeps pressuring keep assuring him of your interest,never cave in ,it is who will be strong and patient   at his position finally wins.

Focus on the other side's pressure, not yours.