Samsung Appliances Repairs Rivonia Road Sandton

I took my washing machine for fixing at the Samsung Rivonia Road branch. The machine had a Motor problem. They confirmed that the Motor was still under 10year warranty. The machine was fixed and I collected and signed that all was ok as the warranty approved. Now i just received a bill of R3500 as they now claim that warranty did not cover the mother board which was also replaced. Should this not have been discussed with me upfront. Why did they give me my machine assuring me that warranty approved the repairs. Something is not right here. Thx