GST Rate on buyback of Leased CAR

I had CAR lease from Tata capital. When the CAR was originally purchased in 2018 the GST charged was 28% + 17% Cess. Now after completion of 4 Years the residual value of CAR is 15% of Exshowroom price. I need to pay this amount to TATA capital, they are charging 28% GST + 17% CESS on residual value of CAR.  I searched on various websites and found that the GST on 2nd hand car sale is 18% + CESS. But Tata is claiming that since the original Invoice has 28% GST and 17% CESS they will follow same.    GST experts can better provide the correct way to apply GST on used car.  Your comments are appreciated.