So in order to earn the “Impossible! Paid off Uncle’s first loan on time.” achievement you need to unlock the fuel pump and then ignore the other tasks, since clients will still come to get fuel but the Loan timer will not start and while they wait you can just lockpick their car for extra cash.
Now need around 5300$, 5000$ for the loan and 300$ for the shelves.but waiting for clients is boooooring so instead of waiting you can use one of these three way to earn more cash$$$ :
1- The RC Race
1- The RC race that gives 20$ if you finish in less than 1m 15s (A total waste of time for the money it gives).
2- The Golf game (The money maker)
The Golf game Right behind the warehouse while it costs 5$ to play it gives you 15$ if you score in less than 5 hits ( so a profit of 10$ and with each game lasting less than 10s this is the best way to earn money)
If you aim right and with the right strength you can get a hole in one sadly it does not give any extra cash
3- The cheezy car flip looting
The cheezy way to flip cars on the road using your Broom, if the cars are flipped they can’t move and you can lockpick all the cars that get stuck.
Be sure to move the flipped car after the entry road to your gas station this way you can still sell fuel, if cars stop spawning or if you have lockpicked all the cars you can just move the car away letting them go and you can put it back in to trap the new ones.
4- The cheesiest of the cheese
So if go to the golf game and you spam the left and right mouse buttons you loose 5$ starting the game but then earn the 15$ you should be only getting if you reach the goal, since it seems that wining and cancelling the golf game gives the same rewards.
Thanks to Zeros for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.