QUESTION: The Gemara teaches that most Toldos of the Av Melachos of Shevi'is -- Zore'a (sowing), Choresh (plowing), and Kotzer (harvesting) -- are permitted during Shevi'is. This is derived from the fact that the Torah specifies certain Toldos as prohibited during Shevi'is (the Melachos of Zemirah and Betzirah, which are Toldos of Ketzirah), which implies that all other Toldos during Shevi'is are permitted. The Gemara uses this Derashah to explain why Charishah (plowing) during Shevi'is is not punishable with Malkus according to one opinion. The Gemara says, "[Only] for these Toldos is one Chayav; for any other Toldah, one is not Chayav."
How does the Gemara derive from this Derashah that one is not liable for Charishah during Shevi'is? The Derashah says that one is liable only for the Toldos specified in the Torah, but not for any other Toldos. Charishah, however, is an Av Melachah and not a Toldah, as the Mishnah in Shabbos (73a) clearly states! (MAHARSHA)
(a) The MAHARSHA answers that when the Gemara uses the word "Toldos" here it refers specifically to Toldos of the Melachos of Shemitah and not of the Melachos of Shabbos. The verses from which the prohibitions of Shemitah are derived make no mention of the Melachah of Charishah. (Although the verse in Shemos 34:21 mentions it, the Gemara here follows the opinion of Rebbi Yishmael (4a) who understands this verse as a reference to the laws of Shabbos and not to Shemitah. Alternatively, the Gemara maintains that if the Melachah is not written in the form of a Lo Ta'aseh, a prohibition, but only as a Mitzvas Aseh (as in this verse), it is not counted as an Av Melachah.)
(b) The RITVA (end of 2b) writes that Rashi's edition of the Gemara had a different Girsa. His text did not read, "For these Toldos is one Chayav; for any other Toldah, one is not Chayav" ("d'a'Hani Toldos Hu d'Mechayev, a'Toldah Acharina Lo Mechayev)," but rather, "For these one is Chayav; for any other, one is not Chayav," without the words "Toldos" and "Toldah." (The DIKDUKEI SOFRIM omits the second word "Toldah," which indeed is the way the phrase appears even in our edition at the top of the page, line 5. This Girsa results in the same implication as Rashi's Girsa.)
According to this Girsa, the Derashah teaches that one is liable only for the Melachos mentioned in the Torah, while for any other Melachah -- Toldah or Av -- one is not liable.
This is also the Girsa of RASHI KESAV YAD (Kupfer), and it is the way the RAMBAM (Hilchos Shemitah v'Yovel 1:3) explains the Gemara. (Similarly, the words of Rashi in our edition (end of the page, DH Acharina Lo) imply that he did not have the word "Toldah" in his text.)