Tues Nedarin 65

The Gemara says that if you made a neder from partaking any pleasure from someone, he should not Matir the Neder unless that person is also present.

The Ran inquires: what happens B'Dieved if he does Matir it when the other party isn't there. There was those who want to say that B'Dieved it's permitted. The proof is from Mesechta Gitten that they worried perhaps the widow, to prove she didn't collect her kesuva yet, makes a Neder on a food that it should be Assur to her if she already collected it, perhaps she'll go to a Chachum behind everyone's back and be Matir it. But doesn't she need to Matir it in front of the heirs (who she made the Neder for.) So, we must say that's B'Dieved it's Mutur.

Similarly, we see that Tzidkiyahu swore to Nevuchadnetzer not to reveal his secret. It bothered him so much to hold in the secret, he had the Sanhedrin Matir it. How can he do so, if it doesn't work? We must say that it's B'Dieved Matir.

(This that the Sanhedrin Matir it L'chatchila, was because the Mitzvah to listen to the king supersedes this prohibition to not Matir behind the other guy's back. Alternatively, it was interfering with Tzidkiyahu's learning and other duties, so it was Mutur to do so.)

However, Ravad held that it's B'Dieved not Matir. In the case of the widow, after the Neder was made, it was no more interest to the heirs whether she keeps her Neder or not (since what was done by collecting it twice was done), so therefore she has the right to Matir if they're not there. Or perhaps, although it doesn't work, she might do it because she might think it works. The same by Tzidkiyahu, perhaps they did do wrong. After all, Hashem punished them for it.