Conservation of energy doesn't prevent this from working. After all, with a turbine powering the wheels, a cart can travel faster than windspeed into the wind anyway.
In this case, a plain sailcart (with no propellor) reduces the velocity of the mass of air that is pushing it by a bit --> reduces the wind's kinetic energy. This enables the cart to move forward (the energy is transferred to the cart) until it's almost at windspeed, where it has a negligible effect on the velocity of the wind.
If a fan is then added at windspeed, it starts to push air back, reducing the air's kinetic energy again. This energy has to go somewhere, either as heat or the kinetic energy of the cart. Since most propulsion systems are not 0% efficient, I'd bet on the cart accelerating (to a speed faster than the wind).
So you see we're not getting free energy from anywhere. Kinetic energy is proportional to the mass involved, and there's a very large supply of moving air available.