Does the first broker have the right to demand a sum of money from me for being the reason I came to know about the apartment, even though the sale was concluded through another broker due to the seller retracting his initial agreement?

If custom dictates that the first broker is entitled to a commission for showing the apartment, then he is entitled to what custom dictates, even if the purchase was made through another broker or directly from the owner. This is because what is known by custom is like a stipulated condition. However, if custom does not dictate this, then he is not entitled to anything.