Is it permissible to take a difference in real estate brokerage commission, knowing that the broker has waived a share of his commission, and I have contributed to providing an amount for the buyer, who is my son?

A father acting as an agent for his son in purchasing land is not permitted to take a commission without his son's permission, even if the intermediary offers it. This is because brokerage requires a clear expression indicating that the work is undertaken for a committed fee. Since the transaction is with his son, the original understanding is that the father's work is a donation, unless the son explicitly permits taking a commission.