Auto substract by comparing the category

Hi CANAPONE,
Thanks a lot for your promt solving the thread.
I have a small problem.
If category was not there and assigned and then we should get 0 (i.e if i assign a category5 in sheet2 and assign amount and if it was not) there.,in the same column or other column and if total money was zero after assigning to some users
is there any possibility of throwing error or populating other column with some error text .

Thanks in advance