When you select a slab, you will see of the slab perimeter is manually done or created by the walls it is bounded to. If you click on this, you can choose to bound to walls. I have a Dutch version, so it can be that my translations are wrong. Sorry for this.
When you then set the components of the slab correctly (the setting to where it must go relative to the bounded walls), then they will cut out the wall.
If it is not yet clear, I can make a picture of it. Just let me know.