You're def gonna need a razorblade, most siccors won't cut it (at least mine won't)
These toadstools are extremely hardy, from my experience you can't hardly kill a healthy one. I just hacked up mine two weeks ago. Cut it back from approx 6" across down to a small stump. I got roughly 20 babies off of it and they were opening up within 24hrs and the mother was opening up within a week.
Since yours is in the display just try to make a clean slice with the blade and within a few days you won't even be able to tell that you cut it. Then you can either LIGHTLY rubberband the baby to a rock or rubble and put it somewhere you can see it and access it should it come loose from the rubberband.