The gemara asks regarding a gett that was given to be effective from now on the condition that he does not return for 12 months, and he dies within 12 months; in the presence of a yaveim, can she remarry someone else within the 12 months since the condition will definitely be met making the gett effective retroactively or must she wait until the 12 months expires. The gemara does not explain what the rationale would be to wait until the 12 months expires. Tosafos suggests that m'doraysa she can certainly remarry immediately upon his death, but m'drabonon we are afraid that people will not hear that he died and think that she is permitted to remarry relying on a gett with a pending condition. The Ran suggests that we are concerned that although he said that gett should be effective from now retroactively, he was certainly not expecting to die and therefore he may have only really wanted the gett to be effective after 12 months (immediately before his death). Based on the Ran, the Beis Shmuel (144:5) points out that it would possibly be a d'oraysa violation to remarry with in the 12 months. The Ramban has another approach, that chalitza is necessary until the condition has been actually met - it is not sufficient to just know for sure that it will be met.
The Poras Yosef quotes a fouth apporach from the yerushalmi. Although he meant the gett should be effective retroactively from today, it will only be effective if he truly does not return. We have to be choshesh for techiyas hameisim, and that he will return. Therefore, it is like any other condition that is pending where we cannot permit her until the condition is fulfilled out of fear that it won't be fulfilled. The korban ha'eida asks, if so, how can a widow ever remarry, we should be concerned that her husband will return. To this the Poras Yosef answers, that by all other women, the death of their husband is matir them. Even if they were to return through a miracle, they are no longer married to their wives. But here where he stipulated "if i don't return for 12 months", and he returns within 12 months after having techiyas hameisim, the gett will not be effective and she will be a widow who is zekuka to the yaveim.
This brings up another issue in understanding how the death of the husband is matir the issur of eishes ish. The gemara in kiddushin 13 searches for a source that the death of the husband is matir the wife. Ultimately the gemara learns it out from a hekesh to divorce. But what aspect of the death is matir her? Is it the act of dying which serves as a matir or is it that she no longer has a husband? The nafka mina would be in a situation where the time of death cannot serve as the matir but the person is dead and will never return. R' Elchonon (kovetz shiurim 2:28) quotes a machlokes between the Ramah and beis yosef (y.d. 267) when a ger dies and leaves behind slaves who are ketanim and cannot be koneh themselves through his death - Ramah holds that these slaves are stuck, since they don't go free with the death of the owner, they can never be freed. Beis Yosef argues because he understands that the nature of death serving as a matir is not an act, rather a continuous status of not having the owner alive. Now, if we understand that the heter of misas hab'al for an eishes ish is an act of dying (like Ramah) then the Poras Yosef is correct in his distinction between a regular almana and our case of condition. But if we understand that death is a continuous state of being where she doesn't have a husband, not a one time act that is matir her for ever, then if we are concerned for techiyas hameisim, we should never be matir a widow to remarry.