This needs to be done on Day 1.
If you have passed Day 1 on your save file, start a new save.
Easy enough, go to the Shop (near Field #20) and spend as much of your money as possible - there are some items as cheap as $400, so those will help reduce your cash balance. You cannot do this by buying an item that you cannot afford, the game will not allow this.
Then, just cycle through your vehicles and press B to hire a worker to do the work for you. This will deplete your cash over time; the more workers you hire, the faster this process.
When you hit negative values, the 40G achievement will pop.
Video for clarification: