In the Midbar, the Mann did not fall on Shabbos. Since, for many, it fell right near people’s door, what would be the problem? Some say that since it came to us in that spot, it is considered Makom Gedulo and it is therefore an Issur De’oraisa of Tolash to pick it up.
I must also mention that blinky’s point is very true. Also, salt can be put down before Shabbos, too.