What is the ruling on one who buys a plot of land measuring 20x20 meters to use as a graveyard, burying people successively, and then re-digs in the same spots to bury new deceased once the old bodies have decomposed, given that the government does not permit burial elsewhere?

The deceased has sanctity just as the living do, and it is not permissible to exhume a grave as long as its occupant is within it, except for a legitimate Sharia reason. If the deceased has decomposed and turned to dust, there is no objection to utilizing the grave plot for burying another person or for another purpose. Exhuming a grave is permissible if there is a necessity, such as burying one deceased person with another before the first has decomposed, or if the deceased has decomposed and turned to dust, at which point it is permissible to bury another person in it, or to cultivate the land, or to benefit from it in any way.