BMW MODERN AUTO, FROM BAD TO WORSE

On the 19 February 2018 I booked in my car at the dealer to check what the cause of the smoke on my car could be. The car was 'diagnosed' and the problems which were found were taken care of. Four days down the line they told me about the car been finished with new parts fitted in. To my surprise when I went to collect the car on the 24 February 2018 the car was worse than when I brought it there. I drove the car for 3/4 km then alerted the dealer about the problem that they claimed had been fixed. They claim that the car was tested and checked by the Quality Controller and all the boxes ticked. I left the car, the following Monday (26 February 2018) the car was in the workshop till Tuesday, on Wednesday I received a call from them (Modern Auto) that the car has a turbo problem and the turbo needs to be replaced! As big as BMW is, I took the car to its home were based on the symptoms the car displayed immediately they should have suspected what the problem was and the "diagnosis" they claimed to have made wouldn't have made predictions. How can a car in a period of 8/9 days have to different outcomes? The car was diagnosed and test driven and it was fine. I come I start the car, it was still the same? days later the outcome is different, the turbo problem was discovered. The car doesn't have turbo, its worse than it was when I brought if from lephalale to Polokwane to repair. Please help what should I do?