Beware of Audi

I recently entered into a purchase agreement with Audi Braamfontein to purchase new Audi. They offered me a trade value on my car which I’ve accepted. We then signed the OTP. They have taken my trade car and lent me a courtesy car while await delivery of my new vehicle. 2 weeks later they come back to me and tell me they can no longer offer the same trade value on my car which is R150 000 difference from the original offer. This is despite them signing a legally binding agreement and still they still have possession of my vehicle. This boils down to pure thievery.