Write Value To Cell When I Delete The Value

I have one cell that contains a function. Sometimes I want to change the value in the cell because the value is used in another function in the sheet.
The problem is that I want the function to reapear in the cell when I press the deletebutton.

Exampel:

1. Cell A1 contains this function.
=Sumproduct((part_number>100)*(Price>10);Stockvalue)
This returns a value that is used in other cells in the sheet.

2. Now I want to try what happens in the other cells if the value in cell A1 is changed.
I select cell A1 and writes the new value that I want to test.

3. If I want the function back I simply should select cell A1 again and press delete.
4. A VBA macro now writes the original formula back into Cell A1.