I'm being threatened for the dealerships mistake

My ride took the wrong details when I traded the opel in on a Nissan. The did the deal according to a screen shot of some registration documents, not on the documents ( settlement letter from the bank) now I'm being threatened because the though the opel was a 2018 Cosmo, when the settlement lettle stats 2017 enjoy and I even told the salesman so. I signed the contract online, when I got to the dealer there was their own documents to sign. I didn't have my spectacles with me so I asked the salesman to read to me and I signed. Now I get blamed because the bought the opel as a 2018 Cosmo, after the settlement letter, the license disc and thd copy of the registration doc was sent to them and I told the salesman when he inspected the car. Now they threatened to go to the police, cancel my deal and contract and seek legal advise if I don't pay R20000............... Because they didn't verify against bank documents...... Now I'm to blame. I have been treatend that they will take it to the SAPD, that they won't settle the car I traded in. They didn't go according to the bank settlement letter, they went according to a copy of a registration document that doesn't even belong to me or my car. So they don't check for accuracy, for *****, for detail nothing. I asked the sales person to read to me what I'm signing as I didn't have my spectacles with me, as I already signed the contract online the previous evening. So for negligence, mistakes, not checking the bank documents, Im being blamed, threatened and victimized