What is the ruling on a merchant selling an item to another merchant based on the request of a customer who deals with the first merchant, and is the money from past sales unlawful if this transaction is impermissible?

The transaction between you and the merchant is permissible if the price of commodity (X) is known, and the customer is an agent for your friend in receiving the commodity from you. The transaction could potentially be an agency from you to him for selling the commodity at its known price, or a sale from you to him. However, in this latter case, he might have sold it before taking possession of it, which is a point of contention if the commodity is not food. As for food, it must be taken possession of before being sold.