If you run out of money in Farming Simulator 19, your balance will be negative. You can't buy new machines, but your employees will continue to work by increasing your debt. When you have little money and your demand for new machines is growing, it's worth thinking about leasing options.
If you can't get the break even point, you can always take a loan from a bank. Open the bank tab in the game menu, where you will find credit options at the bottom of the screen. The loan is taken in "portions" of $5,000 each, and you can return it in the same amount. When you have taken out a loan, you will have to repay the bank - about 0.56% per day. This is not much, but with a $1 million loan it will be more than $5,000 per day. The credit debt is being collected every midnight.