How can I decrease the torque of a stepper motor?
Current is controlled by Vdd , simply reduce  till it slips but your acceleration must be low.

Because if you don’t know inertia reactions, the force can be exceeded with no load. There is an advantage here to use microstepping , because the voltage is more sinusoidal per pole and thus lower torque and greater sensitivity.  

Make the inertial force less than the impact. Then by acceleration and max velocity, minimum impact is achieved but then you lose position tracking.