1. The "kickback" you mentioned belongs to the agency fee stipulated in the contract law and is legal income. Since you have an agreement, you introduced the buyer to him according to the agreement. Of course, the other party should give it to you and has no right not to give it to you. 2. The IOU of B is of course valid. 3. Both A and B can be regarded as performing their duties, and the terms they talked to you and the IOUs they wrote to you can be regarded as ultimately belonging to this store. Therefore, this iou is valid. You can directly ask the owner of this store to give you money. So, if AB insists on not giving you money, you can find a lawyer to sue the shopkeeper.