Take your best car(most likely its in slot 1 in your parking garage). Take it back to your shop. go back to the parking garage move to slot (or space) 2. Exit and re-enter the game reload to the same saved game go to the parking garage and it will be duped. NOTE: Before trying this make a copy of the saved game just in case!

Note: This procedure involves changing game files so create a backup copy of the files before proceeding. Open the game folder and find the save folder that you want to get more money for. Open it and copy any PlayerCar file desired and rename it to a number you do not have. If you have a file such as PlayerCar. (for example, PlayerCar1.2) and it belongs to the car you want to copy (for example, PlayerCar3.4 belongs to PlayerCar3), change is the number before the decimal point. Then, start your game, load your save file, and go to the car lot. You will find your copied car there. Choose it and sell it. Repeat this process for as much money as needed.

Alternately, copy the "PlayerCar1" file and rename it to "PlayerCar2". If you need more money, you can keep going up in the car numbers such at "PlayerCar3", "PlayerCar4", etc. Keep increasing the numbers, but make sure to keep the same name as "PlayerCar".