Is it permissible for me to take a commission from a real estate office without the knowledge of my relatives who bought land through me? Is it permissible for me to take a commission from my purchasing relative in addition to half the price difference from the real estate office if land is sold to him? And is it permissible for me to offer the land to another relative and take my commission only from the real estate office?

There is no harm in taking a commission in the aforementioned scenarios. The transaction is a brokerage, which is mediating between the seller and the buyer. It is permissible to take a commission from both parties or from one of them, with the knowledge of the one from whom the commission is taken. The buyer's knowledge of the commission taken from the office is not required in the first scenario. In the second scenario, if the commission is taken from both parties, then each party must be aware of what was taken from them, without necessarily knowing what was taken from the other party. In the third scenario, there is no harm in taking the commission from the office only. The fact that the commission is the amount exceeding the set price does not affect its permissibility, based on the statement of Ibn Abbas: "There is no harm in saying: 'Sell this garment, and whatever it exceeds such and such is yours.'"