You can't sort on a summary field as you have found out. But you should be able to define a calculation field using Sum that computes the same total and use that for your sort.
Are you trying to sort the portal records?
If so, not only can you not sort on a summary field, you can't sort on any field not from the portal's table occurrence. But a calculation field using Sum defined in the portal's table should do the trick, though it won't be fast given the fact that it's an unstored calculation.
Yes it is a portal I'm trying to sort on. I tried creating a field xSum in the products table, that was calculated as Sum(LineItems::ExtendedPrice) and it did return anything.
Sum, from the parent table should produce exactly the same total as a summary field in the related child table.
You did use a calculation field and not a number field with an auto-entered calculation using sum?
Nice, my fault, did a number thing not a calculation, thanks a lot.