What is the ruling of the Sharia on a father disinheriting his children from his first wife from the legal inheritance and bequeathing all his property to his son from his second wife?

If the father decides to sell his property while he is legally incapacitated, or if he proceeds with the sale while mentally deranged, then the transaction is void. If the sale occurred while he was fully competent, but involved favoritism towards one of the children, it is also void, due to the obligation of equal treatment among children in gifts. However, if the sale occurred while he was fully competent and was free from favoritism, then it is a valid contract.