Change Cell Value and Subtract the cell value in another sheet

Dear All,

I have two worksheet with the following data,


Sheet 1


Invoice Number Qty
Invoice-001 6


Sheet 2


Invoice Number Qty
Invoice-001 1

Now what I want is if the user Change the Qty of Invoice-001
in Sheet2, e.g. Change from
1 to 3. Because the difference from 1 to 3 is 2. Then I need to
subtract 2 from Qty of Invoice-001
Of Sheet1. If the user change the Qty of Invoice-001 of Sheet2 from 1
to 0, Then I will add 1 to
Qty of Invoice-001 in Sheet1. How can I do that?

Thanks