Does the first seller deserve a commission for a second commercial shop rental transaction that was not previously agreed upon with him, after he had introduced the tenant to the second seller for a first shop rental that did not go through? And what is the amount of this commission if it is due?

Brokerage is a ju'alah contract (a contract for a specific service or task), and the broker is not entitled to a commission until his work is completed. Therefore, if someone guides you to a property for rent and it is subsequently rented, he is entitled to the commission. If the commission amount was not agreed upon, he is entitled to a customary commission ( 'umūlat al-mithl). However, if you were guided to a property but it was not rented, or if you were guided to a different property than the one initially shown to you, the broker is not entitled to a commission for that. This is unless the broker did not guide you to a specific property, but rather guided you to a person who owns properties for rent, and you agreed to pay him a commission; in this case, he is entitled to the commission.