Car damaged while replacing the part

I took my car in for service in June 2023 in August I had an issue with car that it was showing an engine warning light and i took it back. They diagnosed the car and told me there is a part that need to be replaced. I ask how the diagnosis did not pick up the fault in June when the car was diagnosed before it was serviced. They struggled to get it car right the whole week. I brought the in on Monday that week and I was called on Thursday to come fetch it. When i got there they told me that there is another problem with the knock sensor so the engine light will come on again. I refused to take the car because to me it seemed like the knock sensor was not part of the diagnosis because it was not damaged then and it was only damaged when they were trying to replace the other part. The service agent Refilwe Mojela called the the workshop manager for me who said I should leave the car and they will contact Mercedes Benz South Africa because actually they should pay for knock sensor. The following day they called me to come collect the vehicle but the knock sensor was not replaced, telling me that Mercedes Benz South Africa said they will only replace the knock sensor if the engine light comes on again. My concern is that even though the engine light is not indicating the fault at the moment but the knock sensor is damaged. I have been asking Refilwe Mojela the service agent to forward me the email or put it in writing that they will replace the knock sensor should the engine light come on again and no response from her. I really need help to resolve this. I want the knock sensor to be replaced or at least something in writing that should the engine light comes on then they will replace it free of charge as they already mentioned that they will replaced it but it must be in writing. Now Refilwe Mojela the service agent in ignoring my emails and phone calls