Frustrated

Good day,I took my car to bmw on the 26th April 2017. They quoted me R45889 for the repairs. I paid the money but a day before collecting the car,I was told there's a knock in the engine. I instructed them not release the car but went ahead and released it. The car didn't make it out of klerksdorp and now they want me to pay R100000 plus for new engine.