Is it permissible to sell personal gasoline vouchers to fuel stations at a price lower than their value, and is the seller sinful if the worker extracts the benefit for himself without the knowledge of the station owner?

If the coupons were gifted to you by the company and you are not prohibited from selling them, then it is permissible to sell them before taking possession, even if for less than their value. However, if the coupons are part of your wage and not a gift, then it is not permissible to sell them before taking possession, though some scholars do not stipulate possession for selling non-food items. It is necessary to avoid selling coupons to a worker if you predominantly believe that he will dispose of them in an unlawful manner.