OK, just spitballin' here to help you save most of your existing slab. It's not beautiful, but it might be functional:
1. Those metal buildings typically have spot footings under the columns, and the walls are not really "bearing". Thus, the rest of the slab does not bear any significant weight. It IS like a pole barn. In the example image below, the gable end walls only have columns at the corners and any door openings.
View attachment 7951
So, since you have 40 acres to play with, you're probably not right on the edge of a planning department zoning setback. Why not shift a building like this one directly/diagonally towards the slab corner closets to us by a foot or so, so that 2 walls and their columns are no longer on the existing pad? For the remaining walls, you can get a concrete saw and cut out squares as large as needed for the spot footings (see red circles above). In the example above, the yellow posts would not longer be on the existing slab - -you'd just build a new/additional perimeter edge for the walls to die into the ground, and some spot footings under the yellow areas where needed.