What is the ruling on taking money from a merchant in exchange for a promise to repeat the transaction with him?

Question:
A man has a shop for selling fish, and he is the one who buys the fish himself from the large merchants. When this man comes to a merchant to buy from him, the merchant gives him one hundred riyals to win him as a customer. The shop owner asks: Is it permissible for me to take this hundred riyals or not?

Answer:
If it is a discount, if he lowers the price for you, whether he lowers it by half or more, that is not a problem. But if he gives it to you as a prize, or gives you a book, or gives you something as a prize, and it is not as a discount, this is not permissible. Because this harms the other shops that do not give prizes; people will be drawn to this person. Also, perhaps he will buy things he does not need in order to get this prize.
So this is an action that is not permissible. As for a discount, if he lowers the price for you, or if he forgives you the entire price, that is not a problem.