Dealing with customers who give trouble during final payment

I had completed the customer’s site in Vizag. The customer, after completing the site itself; he paid the first payment and then put a hold on second payment. He then started saying that I’ll pay you next time. Or else give me a discount on the overall quotation again. Firstly, when we start the project itself, we will finalize the quotation. How much it is flexible and how much it is negotiable. And some customers expect negotiation after the completion of the work too. He is just that kind of person. Expecting negotiation in the last. 

That time I spoke with the dealer and gave him 1000 discount on the overall quotation of 75000. And finalized the payment. So, with some trouble I finally closed it. 

A challenge situation where a strong response is not yet there - may require a combination of solution thinking and human-centric thinking to deal with it