Do not focus on the slat alone, you have to keep the whole system in mind. Slats generate lift ahead of the c.o.g., but they also deflect the air downwards (Dumping velocity). The incoming air thus hits the wing-element at a lower (or negative) effective angle of attack.
Look at this image from A.M.O. Smith's paper "High lift Aerodynamics"
Several things are happening:
- Lift generated by the slat (reduces pitch down moment)
- Suction peak of the wing gets reduced (increases pitch down moment, as there is left lift ahead of the C.O.G)
- Lift of the wing gets reduced (depends on exact effects)
The net moment delivered by the slat is influenced by its effect on the lift distribution of the wing, and therefore rather limited.