Weelee wants to avoid honouring a written offer to purchase

I received an offer to purchase (OTP) from Weelee for R185 000 for my vehicle. Since I owe more than R185 00 to my bank, I had to make special arrangements with them because of the shortfall, which took three weeks. At last, Weelee could collect my vehicle and make their payment to my bank. However, Weelee then told me they wanted to make an offer of R158 000 and NOT R185 000 and that the amount on the OTP was a typing error. This "typing error" was not picked up when they sent me the original OTP, it was not picked up when I sent them a copy of my acknowledgement of debt with my bank, it was not picked up when I also sent them a copy of their own OTP, and it was not picked up when the bank sent them a letter of acceptance, four times. Now Weelee wants to avoid honouring their initial offer.