Are the discounts you receive from suppliers, due to your long-standing work with them, and which you take for yourself without the client's knowledge, permissible or forbidden? And if forbidden, how can the taken amounts be returned without the client's knowledge?

Whatever advantages or discounts the agent receives in the matter of agency, the original principle is that they belong to the one who appointed him; this is based on the Hadith (definition) of Urwah ibn al-Zubayr (may Allah be pleased with him). Therefore, any difference in price you obtained when purchasing those items rightfully belongs to the client. Thus, you are obliged to return those amounts you took to that person, and he must return the rest of the money to him. The money can be returned to the client without his knowledge by applying it to any expenses the property needs or by deducting it from your commission.