Is the breaking of an oath considered to have occurred, and does it necessitate an expiation, if a person prayed forgetfully in a room where he had sworn not to pray, and then continued his prayer after remembering during it, interpreting that prayer takes precedence over the oath?

If someone vows not to pray in a specific place, this vow is considered a permissible vow. He then has the choice between fulfilling it or not acting upon it, along with offering an expiation for breaking an oath (kaffarat yamin (definition)). If he prays in the aforementioned place forgetfully, no expiation is required of him. If he remembers his vow while performing an obligatory prayer (salat al-faridah), he is not permitted to interrupt it and must complete it. No expiation for breaking an oath is incumbent upon him because he is excused in starting and continuing the prayer.