How do I remove or cut a sewer pipe at edge of our basement slab?
You are thinking about this way too much.  If you aren't using it anymore and the concrete is coming up the people doing the french drains won't think twice about taking care of it for you (for free) if they are reputable.  It is literally one whack with a sledge hammer that they will have available.  There is no use sawing through it or whatever if it is going bye bye.  They will simply remove it as they would the concrete around it.  Cast iron doesn't do well with a hammer.  The PVC on the other side can be cut with any kind of saw.