Have the dispenser display a code for a $2 future credit. Or to be redeemed for cash at a later time somewhere cash transactions with humans exist. Or just use a standard vending machine module and give change with a restocking employee.

The market will decide if it's more expensive. Customers exist who will commute to another charger so they can pay cash. If the money left on the table is more than the cost of the cash module and attendant, it will be adopted.