Is it permissible to benefit from the money that the mother gifted to her son, knowing that she favored him over his siblings in the giving? And is he obligated to return it to her account without her knowledge? And if he returned part of it, is it permissible for him to reclaim it later if the mother decides to equalize the children in giving?

The fatwa (definition) we follow stipulates the obligation of equitable distribution among children in gifts, which is the view of the Hanbalis. However, favoritism is permissible for a valid reason, such as the child's need, chronic illness, large family, or engagement in seeking knowledge, or if the other child is an unrighteous person (fasiq) or an innovator (mubtadi').

If the favoritism is not permissible, and your siblings do not agree to it, then you must return what you were favored with to your mother, or share it with your siblings. It is not permissible for the child who was favored to take the excess; rather, he must return it during the lifetime of the unjust oppressor and after his death, or share all the wealth justly with his siblings. It is permissible to return what was favored by depositing it into the mother's account without her knowledge, but it is not permissible to take it from her account afterward without her knowledge.