Rav Zvid's explains the argument between R' Yochanan and Reis Lakish in the context of a woman's "sliach l'kabalas kiddushin." As Tos. explains, the problem is that the woman didnt "tell the shliach".
Since telling the agent is an effective method of nullifying the agency, why is the bitul of agency by the case of Gittin a question on Reis Lakisk? Afterall unlike the case of Terumah and Kiddushin (which Rashi emphasizes the ability of voiding the agency both "b'fenev" and "lo b'fenev" ) the bitul by Get is through direct contact with the agent!
Thanks and T'zchu L'mitvos
Yossie, NY, USA
You have asked the Maharsha's question on Tosfos (DH "Lo Kidshah"). The Maharsha explains that Tosfos holds everyone agrees when the woman herself retracts the Shlichus that it is effective. The argument between Rebbi Yochanan and Reish Lakish starts when the woman herself does not tell the Shliach, but she sends a second Shliach to tell the Shliach. In such a case Reish Lakish holds that the Bitul is ineffective, which is why the Gemara asks from Gitin that we see a Shliach can effectively tell the first Shliach that the Shlichus is no longer valid.
Kol Tuv,
Yaakov Montrose