Is it permissible for a son to ask his father for a plot of land as a gift, or to buy land from him at its original purchase price, provided that he gives the money to his sisters?

It is permissible for a son to buy some of his father's property at the current market price. The default assumption regarding Muslims' transactions is validity until proven otherwise. If the father intends to defraud by selling his son's property for a paltry price, he is sinful. However, a father gifting to some of his children to the exclusion of others is not permissible; he must be fair among his children in gifting, unless there is a reason such as illness, need, or poverty, in which case there is no harm in singling out some over others.