What "barter system" had Dan developed?

Dan bartered his services as a physician for gas.... that was the only way he had of making a bit of money.

Dan had evolved a barter system for his services. He charged a gallon of gas, if the patient had it, for house calls. Most families had somehow managed to obtain and conserve a few gallons of gasoline. It was their link with a mobile past, insurance of mobility in some emergency of the future. Sickness and injury were emergencies for which they would gladly dip into their liquid reserve. Dan made little profit. Perhaps half his patients were able and willing to pay with gasoline. Still, he managed to keep the Model-A's tank nearly full, and on his rounds he was continuously charging batteries.