Billed for "cancelled" trip after pick up and payment

a Trip on Wednesday 10 January 2018 turned sour for me after the driver Kgaogelo picked me up, dropped me off and received payment in cash from me. Kgaogelo was very friendly and professional and even told me that I was only his fifth trip for the day and that work is very slow. I even rated him a 5. I could not believe it when I then received an e-mail notification that I owe R70.00 for the trip as the full amount was not paid!! The amount payable was less than R70.00 and I paid Kgaogelo R70.00 and told him to keep the change. After logging into the APP I noticed that the driver apparently "cancelled" the trip on his side!! This after he made the trip and was paid. Trying to log a query about this on the APP was unsuccessful and just caused me to be debited with the amount "payable with my next trip". I am unable to find an e-mail address for Uber to request them to track this drivers trips and they will then see that he did make the trip. I will not pay out of principal for somebody trying to pocket a measly R70.00 on the sideline.