I am trying to figure out how to keep a running total on something on one sheet, and have an input cell on another sheet that modifies the first one.

Example,
Sheet2!A2 = sum(A5:A7)

On sheet 1, I want to create a cell that will deduct from Sheet2!A2, and then go blank, so I can input something else there to deduct from Sheet2!A2 when I need.

Is there any way to do this, or is this thinking outside of Excel's capabilities?