What is the ruling on concealing additional features and discounts in the broker's account from the buyer, and must the price difference be returned to the buyer if the purchase was incidentally made with a less expensive currency?

If you are acting as an agent for the buyer, you must inform him of the true price at which you bought the item, and any excess amount over that price must be returned to him, unless he has authorized you to keep it. This is supported by the hadith of Urwa ibn al-Ja'd concerning the purchase of a sacrificial animal.