Hertz erroneously charged $x, and as a result caused damage in excess of $x.

It doesn't matter who pocketed the credit card fees and exchange rate difference. All these costs are the direct result of Hertz' mistake, so Hertz should pay.

Making unauthorized transactions makes you liable for all the costs that result from your error. If that exceeds the amount you erroneously charged, tough luck, but that's the price you have to pay for your mistakes.