Is it permissible for my wife to take money as a loan from her sister's husband's wife, or to transfer money to her brother's wife so she can give it to my wife, knowing that I swore an oath of divorce that I would not transfer money to her? And is it permissible for me to retract this oath?

Swearing by divorce (al-halif bi al-talaq) has the ruling of a conditional divorce (al-talaq al-mu'allaq). The oath is broken (al-hinth) if the sworn-upon action occurs, according to the intention of the one who swore. If you intended that the money should not reach your wife by any means, then you have broken the oath if it reaches her by any means. If you intended only that it should not be transferred to her directly, then you have not broken the oath if it reaches her by a different means. If the swearer breaks the oath, his wife is divorced according to the majority of jurists. However, others hold that an expiation for an oath (kaffarat yamin) is required if the intention was to threaten. The husband cannot retract an oath of divorce according to the majority of jurists, although Sheikh al-Islam Ibn Taymiyyah held that it is possible to do so. One must be cautious about swearing by divorce due to the dangers it poses to the family.