Yes. The forces don't balance out, as the sail also redirects the air back towards the fan, giving a net force in the direction the fan is blowing.
This exact mechanism is applied in jet engines. It's how (some) commercial airlines slow down, by putting a metal shield behind the jet engine redirecting the exhaust forward.
It's as if you put a bent tube behind the fan, which directed the air around 180 degees (which would push the cart along) only much less efficient as most of the air leaks around the side.