Lechem mishna, lechem yomaim, in the story of the mannah (Shemot 16), is about the Children of Israel’s responsibilities actively to receive God's bounty and His blessings.
It is not enough for the people to keep the shabbos, they must also prepare for the shabbos so as to make the keeping of the shabbos practicable. The story of the mannah is not about whether a Jew may carry on the seventh day but about how the Jew ought to gather a double portion on the sixth. The institution of shabbos is partly about what is prohibited and partly about what is commanded.
The Children of Israel have a responsibility to make it so they can keep the shabbos. As such they have a responsibility to organize their affairs around the continual production of a weekly surplus that is then immediately consumed every seven days. The society establishes a rhythm of production of surplus and then consumption of surplus – every seven days, every seven months, every seven years, every seven times seven years.
When God promises there to be bounty to keep the laws of sheviit what He is saying is if you build it, I will come. His promise for sheviit mirrors His promise for mannah on shabbos. The anticipation of complaints about the impossibility of keeping sheviit is similar to the Children of Israel complaining to Moshe right out of the box after the Red Sea about their not having food in the wilderness.