There are a couple of ways to go about earning money quickly. Once you unlock the Cafe Theatre, you can do some renovations on it to start the income coming in. At first, income is very slow, but as you perform renovations, it increases. You can also perform Cafe missions, which also greatly increase your income.
There are other Cafes spread around Paris which you can renovate. Again, these unlock more cafe missions which increase your income, and which themselves often have significant cash rewards. (I've seen up to 25,000 Livres).
There are also Paris Stories which have cash rewards. Some of the harder (5 diamond) Paris stories have large cash rewards, such as 25,000 livres.
All in all, there are plenty of ways to earn money, and it really picks up as you move through the game. Investing in renovations early on is a great idea, since they will pay you back infinitely.