Responsa על נדרים 42:3

Q. Certain Jews received permission from their overlord to imprison and torture their fellow Jews in order to extort money from them. They threw a few Jews into prison and, by threatening to kill them, made them bind themselves by a herem to pay a certain amount of money to their captors. Must the captives fulfil their promise after they are released?
A. No, the herem is not binding upon them since they accepted it under pressure, and since they probably thought that the threats of murder would not be carried out, that the overlord would probably not agree to murder, and that the captors themselves would be afraid to commit murder, a sin punishable by God and man. However, in order to be doubly certain, and for the sake of appearances, the captors should be asked to free the others from the herem; and knowing that the herem is not binding anyway, the captors should not hesitate to do so. But, if the captors refuse to free the captives of the herem, the latter are free from obligation anyway, and need not even seek absolution by a scholar.
SOURCES: Pr. 595, 938; Mord. Gitt. 395; Tesh. Maim. to Haflaah, 7; Agudah Sheb. 14. Cf. Weil, Responsa 53; Isserlein, Pesakim 73; ibid. 252.