Bnei Yisrael were given hok umishpat at Marah.  Rashi explains that they were given a few mitzvot, including Shabbat. Afterwards, when they got to Eilim, they were given the mann.  Why then, does the piyut of Dayenu in the Haggadah shel Pesah say "Ilu he'echilanu et hamann" before "Ilu natan lanu et HaShabat?"