What is the ruling on Zayd taking the excess amount, 500 riyals, without Amr's knowledge, given that the agreement between them was for Zayd to sell the land for 2000 riyals, and whatever exceeded that amount would be his, in addition to 2% of the sold land's value?

The described scenario is not religiously problematic. If the owner tells the broker, "Sell this garment for ten, and whatever it fetches above that is yours," this is permissible, and the broker is entitled to the excess. This is because Ibn Abbas, may Allah be pleased with them both, saw no objection to it. Also, the broker is acting with the owner's permission. The broker is not obligated to inform the owner of the land about the extra amount.