I need to write an expression which gives me Sum(Sales) and this should change only if I select Region, else it should remain the same.
For. e.g. if there are 3 regions with sales of 100 units each, then initially without anything selected the expression should show 300, if a region is selected then it shows 100. After this, even if any more different filters are applied, it should show 100 only.
Not entirely sure what you are after, but I think the direction you are looking for implies setting a variable on the selection of region. You then use the contents of that variable in a set expression. The variable only changes when you make a selection on region, so other selections are disregarded.
Depending on what you want the expression could be something like:
