You dont need the stone slab anymore when you have given it to the fisherman. The stone slab was only for the fishermans chair. So no worries about it Unless im missing something about that item in general, its really only meant for the fisherman.
in v0.72.002, the slab seems to be needed for the wall in the snake lady's house, but im not exactly sure what item the fisherman needs to swap the slab for,
