What does the subject of Shemittah have with Mount Sinai? Were not all the commandments stated from Sinai? However, [this teaches us that] just as with Shemittah, its general principles and its finer details were all stated from Sinai, likewise, all of them were stated-their general principles [together with] their finer details-from Sinai.
Yet, in all other mitzvos (Shabbos, for example), one who violates a Tolda of an Aveira has still Biblically violated that Aveira. However, one who violates a Tolda of Shmitta did not.
What does Rashi mean?