Make date change affect another cell numerically

Attached, I hope, is a small receipt for which I and want to know how to have a date change in one cell (F2) change a dollar amount in another cell (E9). I want the amount in cell E9 to be reduced by whatever amount is in a third cell (F4).
What I hope to get is that whenever a date change is entered in cell F2, E9 (the balance) will be reduced by whatever payment amount is in F4.
This is outside of my familiarity with Excel. Yes, I can do simple math between cells but not using a date. If something more detailed would work, I would need a better understanding of the formula to do it myself.
I would appreciate any help...
Thank you,
Sharon2287