Formula Set Up Challenge

Excel Forum,

Thank you for your time reading this post. I am grateful for your help.

I am struggling to find an efficient way to set up a formula that will achieve my goal.
I have a series of cash flows timed by quarter. I need a formula that will add up my cash flows each quarter and once a unit sells it is no longer included in the cash flow stream retroactively.
Attached is a spreadsheet with an example of what I am trying to accomplish.

For example, in quarter #2 (row 18) no units sold so I just sum the cash flows. In quarter #7 (row23), 2 units sell and I need to add up all of my quarterly cash flows again except I need to exclude the units that sold. In other words, the cell labeled "sale" needs to trigger additional arithmetic.

Thank you for your time and guidance.