Super Group Dealership - Sale/Trade-in mess

I recently bought a vehicle, Volvo V40, from a Supergroup Dealership - Auto Baltic Eastrand. About 2 weeks after the sale agreement was concluded I was contacted by the dealership to tell me that the deal had been structured around the incorrect trade-in reg year. The difference in trade value amounts to R40 000. Dealership wants me to add this amount to my financing agreement or they could just cancel the deal, uplift the V40 and return my trade-in. Why am i being held responsible for their mistake? This should have been picked up when they requested the settlement value on my trade-in, or the NaTIS document, by both the sales people and also by the person arranging the financing. They've failed to settle my trade-in which means that I now have 2 vehicle's debt that I am responsible for and if I default on either it will affect my credit rating. I have also revisited the financing agreement and see that the V40 reg number captured by the financing person at the dealership is also incorrect. Will I be held responsible for this as well??

Trade-in Description: VOLVO XC60 T5 EXCEL POWERSHIFT 2012 Registration number: BR28RSGP Engine number: B4204T***8 VIN number: YV1DZ475BC***

Vehicle bought from Auto Baltic Eastrand Description: VOLVO V40 T5 EXCEL/MOMENTUM GEARTRONIC Registration number: DR79MXGP Engine number: B4204T*** VIN number: YV1MV40HDF*** Registration year: 2015 Manufacturing year: 2015