Billed after selling car

We sold our car on the 11th of January 2016. We gave the new owner time to register the car on his name, when this was not done we deregistered the car off our name on the 3rd of March.<br> As we new that etoll might be problematic we checked every 2 weeks what our account was. On the 15th of March Etoll offices assured us the amount was less than R50. Now suddenly when we went in the amount is suddenly R516.44 ! This is unacceptable as I did my part to ensure what the balance was. I will not pay for ETolls inability to provide the correct amounts in a timely fashion. How can you expect people to pay within 7 days, if you are not even aware of transactions that happened one month ago!