Until it does, though, the air from the fan will bounce off the sail and go (mostly) backwards, propelling the craft forwards (for the same reason, albeit much less efficiently, as if you had no sail and just pointed the fan backwards. You've basically made a thrust vectored device, badly).
A sail is used to create drag. If the air from the fan hit the sail and dispersed perfectly perpendicular to it, then it wouldn’t move the craft at all.
The more realistic outcome would be that the sail did not disperse the air with perfect efficiency, and the craft would just move backwards.
What you’re describing would require redirecting the flow of air backwards, like a curved exhaust. Which is really just a less efficient version of turning the fan around.
It should push the fan and the sail apart, which could result in a structural failure.