To perform this trick you have to save your game and exit instead of driving away when you have finished refuelling your truck at a service station. When you then reload your saved game and drive away from the gas station the the game will have forgotten to have charged you for the fuel.
Hint for Police and Fatigue:
This hint is to disable your Police ticket on various points like Red Single, traffic offence, over speeding, wrong way etc. and Fatigue during long driving, just go to
1.C:/UsersUser nameDocumentsEuro Truck Simulator 2
2.Go to Profile folder
3.Open config.cfg file in notepad (right click on file Open with then chose program and then hit notepad)
4.Change the values of uset g_fatigue "1" and
uset g_police "1" to uset g_fatigue "0" uset g_police "0"
5.then save the file from File menu
Now start the game and enjoy without police and fatigue...