Is it permissible to give money to my father as the price for my house, so that he can buy a house for my brother?

As long as the man gave you the money, believing it belonged to your brother, it is not permissible for you to take anything from it, because you are merely an agent entrusted with delivering this money to your brother, and an agent is trustworthy. However, there is no harm in informing your brother of what happened, as he might forgive you.