Thurs Nedarim 67

If the husband makes Hafara and the father is Mikayim, so the Neder stands. However, if the father is Matir Neder for his Hakama and is Meifer, we do not combine it to the husband's earlier Hafara, since they weren't done at the same time.

Ran explains that it doesn't have to be done simultaneously, but it means it cannot combine with a Hakama in the middle. The Ramban holds that now the husband may Maifer a second time to combine with the father's. The Rambam holds that he lost his ability to be Maifer, since he already id it.

The Ran asks: Why don't we say that when he Matir Neder on the Hakama, it was if the Hakama never happened, so they should combine? We see a precedence to this: if a man marries a wife on condition she doesn't have Nedarim, Even if she had Nedarim, if she's Matir Neder we view that the Neder never happened and it's a good Kiddushin.

The Ran answers: over there, Kiddushin is an entity by itself. Therefore when she was Matir Neder, we say the kiddushin takes effect retroactively. However, here, the husband's Hafara is nothing by itself. Therefore we cannot say it takes effect retroactively, but you need to combine it with what happens after the whole Hakama and Heter. Therefore we cannot say they combine retroactively.