Deliberately ripping customers off

They knowingly sold a faulty headunit to me and when I disputed the matter with them they not only refused to rectify their wrong doing but also told me "tough" their customer service is appalling! More than that they still owe me R1800 as a refund for over charging me. I am yet to receive my refund

Now they are telling me to pay R900 to them to resolve their problem.

I am of the opinion that they purposely sell faulty units to customers in order to generate additional "repair ticket" fees