What is the ruling of Islamic law concerning a father taking back a gift he gave to his children, knowing that he initially wanted to take it back as a gift conditioned on a monthly payment, and then wanted to go back on that again? Is it permissible for him to threaten his children with fraud or by instigating discord among them?

A gift conditioned on a monthly payment (a gift of reward) takes the ruling of a sale. If the amount is known, the contract is valid and binding unless the father cancels it. If the amount is unknown, the contract is void. A father's favoritism towards some children is an injustice if there is no legitimate Shar'i justification for it, but it does not permit preventing the father from disposing of his money, nor does it permit placing him under interdiction except through a judge. The father should be advised and guided in the best possible manner.