Scammed to do a service

We took our car (dodge journey) in as it has a jerking problem when you drive or feels like it will cut off when you stop, the advisers at Jeep said the car needed a major service to sort out the problem, when given a quote of R16000+ for the work to be done, I explained that the quote is expensive for me and ask Christa if that will really solve the problem? She said yes it will and even went on to say if it does not solve the problem I will be paid back the service fee. I then gave her a go ahead because I really wanted the problem to be solved. To my surprise on the same day I took back the car the problem was still there we notified the dealer and I was asked to bring it back in. Now I'm being told that I must pay an extra R5000+ for something else that is now apparently causing the problem. This is so unfair for me as I was not ready financially to take my car in for a service but only agreed to it because I was told it will solve the problem.