who pays?

hi all, i had a problem with my cluch, and was told i needed to change the dual mass flywheel, cluch plates and cylinder, i supplied the geniune parts (got them at a discount as one of my relations works for fords) to a good local mobile mechanic, and he fitted them, after driving the car for about 60 miles, car was leaking oil, local garage said it was the rear driveshaft seal, which must of happened when new flywheel/cluch was fitted, took it back to mobile mech, who said he will change seal at no charge to myself. when he took the gearbox off, it was the cylinder i supplied that was leaking, and not leaking from rear seal, as it was myself who supplied the clylinder, and i saw it fitted correctly, mechanic said i will have to pay for the faulty cylinder to be replaced, as he had fitted all the parts i supplied correctly. he has said he will discount the price as a goodwill gesture, but do i have to pay for this work to be done again.