Law of Priority Between Two Buyers

I made an agreement with a person who needed money to buy an apartment from him, and I wrote in the agreement that I can sell to a third party. I searched for a third party and found one through a broker, and we shook hands, so the parties agreed, thank God. After I closed with them, my seller (with whom we have not yet completed the sale) brought someone at the same price and claims that I have long planned for this person to buy the apartment from you through me, and I would earn the brokerage, and now I am at a loss, and it would have been at least proper from a decency standpoint to notify me, etc. So the question is: where do I have more of a duty of decency and fairness — towards my seller or towards the broker and buyer I closed with?

Shalom and blessings 

The answer of the Dayanim of the Beit Hora'ah:

Simply put, there is no acquisition here because a handshake is not considered an acquisition until there is a contract; this is not even a situmta, and one can go to whomever they wish.  

However, if all the details were agreed upon between you, then there is a lack of faithfulness to the one with whom you shook hands.