Unfair treatment from Car service city Constantia and Nissan Clearwater.

I sent my car for a service at car service city Constantia Kloof, the car later had an oil leak. I took back the car and I was told to pay for a damage to the engine that was due to an oil stavage which was a a result of an oil filter which was fitted by them during a the service. Car Service City then told me that there is no way the car broke down because of the oil filter but due the oil pump failure. I took back the car to Nissan and Nissan refuse to fix the car telling me that the car was serviced at a dealership that is not Nissan approved. I am therefore left with a damaged engine and two companies refusing to take liability for the car.