What is the ruling regarding a father transferring ownership of the house to his two sons through a sale-and-purchase agreement, and transferring ownership of a vacant plot of land to his four daughters, and what is the burden of sin resulting from that?

A father is not permitted to favor some of his children over others in giving gifts, unless there is a legitimate شرعي justification. What the children provide to their father, such as financial support or gifts, is not considered a justification; rather, it is their duty. It is the father's duty to be just among his children in giving gifts, and he is not allowed to favor some of them unless there is a need, a disability, a large family, engagement in seeking knowledge, or if the child is impious, an innovator, or uses the gift for disobedience. What was mentioned about the fictitious sale to the two sons without the daughters is not permissible if there is no legitimate شرعي justification for it.