So the requirements feature should allow more flexibility than just reverse the sign.

The issue is that right now you can only perform calculation on formula of fields that you've already selected with the @ symbol.

Suggested implementation Solution :

So I guess the idea is that when we add a new formula with the @ sign we should be able to access to all the fields for given table and not only the fields we've selected for extraction.

You will achieve what you want with this, and this will also cater the needs of other data transformation,

since in the formula we can put Excel formula, we could also do nice things like =Concatenane("Cust No";" ","Cust Name")

no, no, no :)

I dont want to create another field that will be visible in the excels table and the pivot tabel - this confuse people. I want to have ability to reverse the sign in the field itself 

This can be done like this :

From Table builder do this :

1/ Click on the ADD FORMULA

2/ Give a caption name to the newly added field : ie : Reversed Amount

3/ Select the field you want to reverse by typing @ then type your formula :

ie :  [@[Amount]]*-1