Wed Nedarim 80

We say that someone cannot Maifer a Neder until it starts. So if there is a condition to be met before the Neder takes effect, hen he cannot be Maifer, because just make sure the condition is kept and there won't be any suffering on her part. Of course, if the condition contains something that causes her to suffer, he can Maifer it immediately.

Asks the Ran: if so, when a woman says she''s a Nazira when she gets divorced, the husband can be Maifer. Why? the condition of the Nazira didn't take place yet, and it's not suffering to remain married, so the husband shouldn't be able to Maifer?

Ran answers: only conditions that in her hands to not fulfill, where she can avoid suffering, we say it doesn't take effect until the Neder starts. However, since divorce is his decision, it's not in her hands not to get divorced and avoid suffering going through Naziros, therefore the husband can be Maifer.