Charged $2,000 for rebooking

Our flight from Florida was delayed repeatedly and we were told we could rebook by the gate agent. I rebooked the flight, but I was not informed of any charges. The agent told me it was a simple rebooking later $2000 worth of charges showed up on my United account now they are claiming they charge the price difference between the flights but no costs were ever mentioned nor did I approve them. What is my recourse here?