A contract for supplying the cars with fuel

Q: We have a fuel contract for the cars of one of the companies with which we have such agreements. The cars of this company come from Riyadh to Tabuk, passing by our station located in Ha'il. An amount of petrol usually remains in the container from which cars are fueled. The driver asks for payment of a sum of money in cash in return for the remaining amount in order to be able to get fuel from the city of Tayma' in addition to paying the road fees. Since the price of oil is 0.37 riyal per liter (r/l), is it permissible for us in this case to price the amount for which they want to compensate us at a less price than that of the contract; which is 0.37 r/l for instance 0.25 r/l in return for payment on credit? The contracting party and the drivers all agree on this. (Part No. 11; Page No. 92) They insist on this and do it in this way throughout the roads of the Kingdom (of Saudi Arabia) which they use in their journeys. For example:- A tank truck is registered to have an amount of fuel as follows:5000 liters of diesel, from which only 3000 liters have been taken according to the capacity of the vehicle's tank. Then the 3000 x 0.37 r/l will be counted according to the contract and the registered amount of 2000 liters in the tank remains.The question is: Is it right and permissible to count the remaining amount of 2000 liters multiplied by the price of 0.25 r/l? For instance: 2000 liters x 0.25 riyals = 500 riyals, to be given to the driver or the person in charge of the car in return for facilitating the task of the driver with regard to the remaining distance of the journey to Tabuk. Note that there is no other contractor of the remaining distance from Ha'il to Tabuk, and that the causes of the price difference from 0.37 to 0.25 r/l are attributed to the fact that we pay 0.25 r/l directly to the driver and that obtainment of money from the owner of these cars will be done on credit, including fees of postage, accountance and fines of delay stipulated in the rules applicable in the owner's company. We hope Your Eminence will reply.