What is the ruling on money earned (5,000 EGP) by a real estate broker who sold an apartment for 105,000 without the buyer's knowledge of the agreement that stipulated that any amount exceeding 100,000 is for the broker?

There is no blame upon the broker for taking the agreed-upon increase with the owner, whether the buyer knows about it or not, based on the owner's statement: "Sell this for such-and-such, and whatever exceeds that is yours." This has been permitted by the Maliki, Ahmad, and Ishaq schools of thought, and it is narrated from Ibn Abbas, who considered it similar to Mudarabah (profit-sharing partnership). Evidence for this is what Al-Bukhari reported from the predecessors regarding its permissibility, and that "Muslims are bound by their conditions."