What is the ruling on a son taking money from his mother, who is unable to perform Hajj due to illness, with the intention that it will become his property after her death, or his children's if he passes away before her?

The preponderant opinion is that whoever is unable to perform Hajj due to old age or a chronic illness from which recovery is not expected, and possesses sufficient wealth for Hajj, must appoint someone to perform Hajj on their behalf with their money. If they die before Hajj, their heir must take money from their estate to perform Hajj on their behalf, whether they made a will to that effect or not. If the mother is still alive, she must appoint someone to perform Hajj on her behalf. If she passes away, the questioner must perform Hajj on her behalf with the aforementioned money, or appoint someone to do so. Whatever remains of the money after Hajj is an inheritance for the heirs. A will is not enforceable for an heir unless approved by the remaining adult heirs. As for someone who is not an heir, it is enforceable within the limit of one-third, and anything beyond that depends on the consent of the heirs.