Since Moshe had told them that the Man would not fall on Shabbos,, how could they take their baskets and go out to collect?
Oznayim la'Torah (citing Seifer ha'Mat'amim): Dasan and Aviram scattered their Shabbos portion of Man in the desert on Friday, in order to prove Moshe a liar. 1
Oznayim la'Torah (Ibid.): Only it was eaten by birds- which explains why the Torah concludes "ve'Lo Matza'u".
What is the significance of the fact that their breaking of Shabbos is followed by the Parshah of Amalek?