As far as my understanding you requirement does not need either nested sorting nor combined fields.
You have to sort the customers based on the sales and want to show the corresponding forecast
For this ......
Just right click on the customer field in column shelf and sort using field (sum(sales))
then drag the customer to filter shelf for creating top parameter using top wizard under filter wizard.
I am also attaching the resulted twbx file , Hope it meets your requirement.
Thanks & regards
REsult.twbx 153.8 KB