2,840. Must a Vow Be Canceled Verbally?

Hilchos Nedarim 13:7

One who annuls his wife or daughter’s vow must do so verbally; if he only does so mentally, the vow is not annulled. However, one who voids a vow need not do so verbally. Rather, he annuls the vow mentally and has the woman do the thing she had prohibited herself. The vow is annulled whether or not she does the thing.

Hilchos Nedarim 13:8

One can annul a vow on Shabbos regardless of whether doing so is necessary for Shabbos. However, one should not tell his wife or daughter on Shabbos that her vow is annulled as he would on a weekday. Rather, he should annul the vow in his heart and then have her do the thing she prohibited to herself.