I need help with a financial balance

Hi,
I want to make a financial balance [=SUM(earnings)-SUM(expenses)] with the following conditions:
1. When it goes below 0 shows a 0 instead of a negative result.
(e.g: Earnings=10; Expenses=15; Real balance=-5; Objective cell=0)

This first part it's easy, I have accomplished this with the following formula:
Objective cell=IF(SUM(earnings)-SUM(expenses)>0;SUM(earnings)-SUM(expenses);SUM(earnings)-SUM(expenses)+ABS(Real balance))

2. If now I get a new earning, it adds to my objective cell, and not to the real balance.
(e.g:
Earnings=10
New earning=10
Total earnings=20
Expenses=15
Real balance=5
Objective cell=5 (so it shows the previous balance (=0)+new earnings=5). The problem is that the objective cell will only be 0 when the real balance it's negative and, when the new earning is introduced, the real balance stops being 0 so the difference won't be added.

I am stuck here, I hope someone can help me.

Thanks for reading!