Uber ****

I requested a ride to work using Uber, I had credit on my account so I paid the difference in cash but Uber claims I didn't pay it. I have been trying to get hold of the customer service but nothing received. I been told that I have to settle the amount for what I don't know because there's no way that I got out of the Uber without paying. I have sent countless messages to clear this outstanding amount but they still haven't responded. I'm not happy with the poor customer service