When you return energy from your car back to the grid you get PAID for doing so. You don't 'give' it back!!
And the whole point is that you get paid more than it cost you to buy it in the first place. You charge up in the middle of the night when demand (and pricing) is low, and then your car returns some of that energy back to the grid during the 7pm peak for which you earn a much higher rate per kWh than you bought it for.