Stefan has shown how to take a surface body in the feature manager and insert it into a new part, and then break its references.
There are a few ways to do this.
I would do everything in an assembly. Place this new surface part in the assembly, or even just use the part that I originally took the surface from. After placing this part correctly with respect to what you want to cut, You can now go to edit part. You will have to create a copy surface (offset '0') in the part that is being edited from the surface part or the origial part itself. Then use this surface to cut the body.
or....
Go to the part you want to cut with that surface and go to insert -> part. Place the part file into the other part.
RFUS