Is it permissible or impermissible for a father to sell and register two floors of his house to his youngest son before his death? And does the youngest son inherit from the remainder of the house? And how can the son seek forgiveness for his father if what he did was forbidden?

If there is no favoritism in the father's sale to the youngest son, then there is no harm. However, if there is favoritism, then he is sinful, and forgiveness should be sought for him.

The ground floor is an inheritance to be divided among all heirs.

One should seek forgiveness for the father and forgive him, whether there was favoritism or not.

If what transpired between the father and the son was a gift, then if it occurred during a death illness, it is a will that is not effective unless approved by the other heirs. However, if it occurred during a state of health and sound mind, then it becomes effective upon death, and the remaining heirs have no right to revoke it.