If someone makes a vow to someone else to become either a shomer chinam or shomer Sakhar for someone else’s property can they do hatarat nedarim for that (assuming of course they inform the person and return the property) even if that other person wants them to be in charge of their property (if they are shomer sakhar this is also assuming they return the money they got as payment)? English language sources appreciated please
Assuming I understand correctly: Abe goes to Ben and says "can you keep my dog at your house this week while I have company", Ben says yes; at Abe's request, Ben vows that he will do so. The next day, Ben changes his mind, goes back to Abe and says he doesn't want to do so, and he'd like to have the rabbis annul his vow. But meanwhile, Abe says no, you are stuck with it!
If that's the case, then Ilfa graciously pointed us to the clear-cut source on this: Shulchan Aruch Yoreh Deah 228:20.
נדר על דעת חבירו אין מתירין לו אלא אם כן הודיעו לאותו שנדר על דעתו (וי"א דאפילו הודיעו אין מתירין אלא מדעתו ורצונו) (מהרי"ק שורש כ"ב וטור ורא"ש) (וכן עיקר)
[Rabbi Karo writes:] A vow made upon someone else's terms may only be absolved if the counterparty has been informed of this. [Rabbi Issreles adds:] And some say that merely informing the counterparty isn't enough; it also requires his consent -- and this is correct.
Rabbi Issreles (of Krakow) would say the oath can't be undone (which would thus be binding for Ashkenazim). At first glance Rabbi Karo (of Safed) would say it can, which could imply it would be permissible for Sefardim, but the Shach comments that this is what Rabbi Karo meant as well -- "informed" meant "informed consent."
אלא אם כן הודיעו כו'. נראה שגם דעת המחבר דבעינן שיודיעו ויסכים ברצונו ודוק:
It is possible that some Sefardi poskim read Rabbi Karo's "informed" as simply "informed" and would allow it ... but as I'm (sadly) not (yet) a guru on Sefardi poskim, I couldn't tell you.