Is it permissible for a customer to take money to buy merchandise from a seller and then sell it back to him? And is selling foreign currency at a deferred price, and obtaining an uncertified check in the local country, permissible or forbidden?

It is permissible for you to go with the customer to purchase the commodity, then take possession of it and transport it from its original location before selling it to the customer, based on the saying of the Prophet, peace and blessings be upon him: "If you buy something, do not sell it until you take possession of it." The principle is that you should purchase the commodity yourself, or appoint someone other than the customer as your agent. However, if there is an urgent need, it is permissible to appoint the customer as your agent to travel and purchase the commodity for you. Then, once he brings it and you take possession of it, you can sell it to him. The commodity would be an amanah (trust) in his hands until the sale is completed.

The agency contract must be separate from the sales contract, and the promise between you should be non-binding. It is not permissible to sell one currency for another except with immediate possession, whether actual or constructive. The solution is to appoint someone to bring the money from Turkey, and then the exchange takes place in your country, or to appoint someone in Turkey to deliver the money, provided that the equivalent currency is deposited into your account immediately.