Ola driver denied cash payment

I booked an Ola ride from Electronic city to the airport. After the completion, I paid 1200 cash to the driver. Later after two days, I got a message from Ola asking me to pay the dues for the ride as they are added to the postpaid account by them. I emailed them and informed them that I had paid cash to the driver. Then I called the Ola support and raised the issue with them. They checked with the driver and informed me that the driver has denied the cash payment. I refused to pay the amount as I was billed incorrectly billed. After that, they started calling me daily to pay the dues. They harassed and used harsh words and even shouted on the calls. I started to ignore the calls after that. Now after almost 45 days, they have sent me a text message stating 'Legal action might be taken for your unpaid dues''. What legal action are we talking about here and is there a way by which I could challenge them and file a case against them?