Kesubos 81b - Takana that yaveim can't sell

The gemara has a discussion regarding a yaveim who is not allowed to sell any of the property that he inherited from his brother since it is all meshubad to the kesubah, unless she gives permission to sell it. The gemara quotes a machlokes whether this takana is that lechatchila he is not allowed to sell, or that even b'dieved the sale will be null and void. Although the gemara rejects the proof cited by R' Yosef, it does not necessarily reject the idea of R' Yosef that the sale would even be void.

Simply speaking the discussion in this gemara is limited to this specific issue as to how strict chazal were when they set up this takana. However, R' Akiva Eiger in a Teshuva (1:129 d.h. v'yesh lomar) quotes from hagahos mordechai that his issue is very much connected to the machlokes abya and rava in Temura whenever the Torah says not to do something, is it binding if he did it anyway. The Hagahos Mordechai proves from our gemara that if one would swear not to sell something and then sells it, the sale is void, just as in our case R' Yosef maintains that the sale is void because he violated the takana d'rabonon. R' Akiva Eiger points out that the concept of "iy avid lo mehani" according to the hagahos mordechai who assumes that this is the issue of our gemara, applies even to an aveira derabonon, such as here where he simply violated the takana. This is in contrast to the Panim Meiros (quoted earlier in the teshuva) who says that our gemara is not a proof that this concept applies to even an issur d'rabonon. The Panim Meiros explains that our gemara is not connected to the bigger question of whether something the torah forbids can be binding bidieved. He maintains that rava's position of "iy avid lo mehani" does not apply to a derabonon, so it cannot be the rationale in our gemara that the sale should be void. Rather, in our gemara the sale is void since the takana was for the zechus of the wife, and since chazal were concerned for her benefit they strengthed the takana that even bidieved the sale should not be binding. Based on this it is merely coincidental that Abayei holds in general that "iy avid mehani" and by us he holds the sale of the land is binding.