Excel Formula to calculate difference between values for certain months

Good day, 

I need advice on how to complete step 2 of my calculation (in col C) as I have not been able to find a suitable solution. 

The attached spreadsheet has information for employees' monthly spending budget - overspending is formatted in red. 

Step 1:

I have done the calculation for Apr-Aug (refer to col C). During these months the overspending and under spending was balanced out to the monthly allowed budget (refer to col E).

Step 2:

From Sep-Mar the under spending is not being calculated anymore, so I am trying to find a way to only add the overspend values to the existing calculation in col C (e.g. budget 700 vs actual spending 750 should only add 50 to col C for the YTD Balance; repeat for each month between Sep-Mar).

Thus, I must check each month if there is overspend, and if so add only the overspending, and if there is under spending then nothing should happen.

Problem:

I don't know how to calculate the overspend from Sep-Mar and add the overspend to the YTD Balance (formula in col C).

I would appreciate any assistance.