To Subtract Part of a Solid Back to an Intersecting Surface

Select the Difference Feature tool.
Set Keep Original to None.
Select the solid that is to be modified.
Select the surface.
The proposed modification is displayed.
Accept.

Left: Original geometry is a slab feature and a B-spline surface. Right: Resulting geometry after the operation.

Note: For the above procedure, the direction of the surface normals points to the part of the solid that is retained. You can use the Change Normal Direction tool to reverse the direction of surface normals.