Is the aforementioned currency trading — which takes place between France and Algeria through the son — considered usury or a salam transaction? What is the ruling on the money generated from it? And how can one deal with the father in this situation?

Trading currencies is permissible under certain conditions. Selling a currency for the same type of currency (e.g., dollar for dollar) requires immediate possession before separation (spot transaction) and equality in amount. Selling a currency for a different type of currency (e.g., dollar for dinar) only requires immediate possession. What your father does, selling a currency for more than its transferred value, is impermissible. The way out of this is to transfer the same currency and take a fee for the transfer as a lease (ijarah) and a loan (qard), not as currency exchange (sarf). You must advise your father, and if he does not comply, you are not permitted to assist him in what is unlawful, and you must stop working immediately. As for what has been acquired, the original capital is permissible for you, but the profits from the unlawful transactions must be returned to their owners, or if they are unknown, they must be given away in charity. Allah Almighty says: "And if you repent, you may have your principal – [thus] you do not wrong and you are not wronged."