Is the aforementioned percentage permissible in return for assisting merchants in increasing their sales on e-commerce applications, which is done by paying an amount to purchase a merchant's product without receiving it, and then reclaiming the amount with a 20% commission?

What the questioner referred to as assisting the merchant in increasing his sales percentage is, in reality, deception of the shoppers. It creates the illusion through these numerous fake sales that the product is popular and that people are eagerly buying from this merchant in large numbers. This elevates the online rating through deceit and fraud. Therefore, this method is not permissible, and it is not permissible to take a fee for it.