In a situation where your cash tank is full, you will not earn more money and properties will continue to generate costs. This is easy to see by the red color of the cash balance and the information that revenue is negative.
In this case, you should buy more properties so you will not lose any more income. There are two ways to increase your current cash tank limit. One of them is to advance to the next level and the other is to upgrade the Banker skill.
Tip: You don't have to worry that if you sell your property for a large amount, you will lose profit. Despite exceeding the limit, the entire sum will go to your account.
Negative (not to be confused with revenue):
This could be due to a bug. In this case contact Customer Support. To not let such thing happen in the future we advise to log in at least once per 3 days to not earn negative income.