What is the ruling on a father selling land to his son on condition that the son perform Hajj on his behalf after his death, if he is able to do so?

A father must be equitable among his children in gift-giving. It is permissible [to differentiate] if there is a reason, such as need, but it is preferable to avoid it and achieve justice. A father giving land to his son without payment raises the suspicion that he gifted it as a way of preferring him, unless the father had sold it and the son was unable to pay the price, whereupon the father forgave him, in compliance with the Almighty's saying: (And if there is one in hardship, then [let there be] postponement until [a time of] ease. But if you remit [it] as charity, it is better for you, if you only knew). If the son was not in hardship, or if the father intended to show favoritism, then the father should take the price of the land from him and then pay it to him or to someone else to perform Hajj on his behalf, or make it a debt upon him to be paid whenever he is able. If he dies before fulfilling it, the debt should be deducted from the son's share of the inheritance. If the father has not performed the obligatory Hajj, he must perform it himself if physically able; otherwise, he must appoint someone to perform Hajj on his behalf.