What is the ruling on buying fuel coupons with Dinars from someone who owns them, then selling them to another party in exchange for a deferred invoice with a percentage of profit or for a service fee?

It is impermissible to sell diesel coupons using either of the two methods mentioned, because that falls under selling goods of exchange before taking possession of them, which is prohibited. These coupons are essentially certificates representing the value of diesel; they are treated as commodities, not as money. Selling them to truck owners for a larger quantity of deferred coupons makes both exchanges a sale before taking possession of the goods to which they entitle.