How to run a motor with lower input power than it's rated for
You say the full rated power of the motor is 850 W, and you want to run it from a source that can put out only 250 W.

You should be able to get the motor to turn, but of course it won't be able to develop the rated torque at the rated speed.  At about 30% of full power, the motor should still be able to do some useful things.

One problem may be if there is a lot of starting friction to overcome.  The motor might not be able to get going at all.  It will just sit there and overheat, or the motor controller will throttle back the current to it.

If you have anything like a viscous friction load, the motor will spin, but not as fast as when full power is available.

Another issue is that the voltage and current you supply have to be something the motor controller can work with.  Look at the power input requirements of the motor controller and see if the voltage and current the solar panel is configured to put out are within the valid range for the motor controller.  If not, that has to be dealt with too.  That would probably require a switching power supply between the panel and the motor controller.  However, by that time it might be better and simpler to just get a motor and controller that are matched to the panel in the first place.