What you must do is play Nissan finance off against another finance company or even a fictitious one. I got my monthly payments down with the same deposit and very similar balloon payment from an initial 850 per moth to 470! Just by saying I had a better deal elsewhere. Oddly enough it was also cheaper over 24 mnths than 36.
Also each pcp finance agreement comes with a minimum future value so if it's actually worth more sell and pay off the balance if it's worth less drop the keys off to the finance company it's there problem.