Usually, it is the responsibility of the car- rental company to get the car back to their workshop in case of a breakdown. However, you need to read the rental agreement. You may write to them and tell them you will pay only for the days you used the car and you may use the support of Consumer Protection should they threaten you of any legal action. I believe they will not take such a step as such actions cost money and takes much time. Also, you have kept yourself protected through written messages to them.