Is it permissible for my brother to buy land from someone who has a loan from the Real Estate Bank, with the intention of selling it and buying another piece of land and building on it with the loan money?

Borrowing from a usurious bank with interest is forbidden by scholarly consensus, while borrowing without interest is permissible. However, it is better to avoid it if possible due to the predominantly illicit sources of its funds. As for borrowing from an Islamic bank, it is permissible absolutely. If the real estate bank lent your brother a benevolent loan without interest, there is no harm in him purchasing land and building on it with that money. But if the bank lent him with interest, it is impermissible for him to buy land or build on it with this ill-gotten money. He must return this money to the bank and avoid paying the usurious interest if he can.