Sort by difference between values in a pivot table

[Terrym76]

I have a pivot table that compares last year's sales to this years. There is a column that shows the quantity sold this year as the difference from last years sales. I would like to sort my pivot table based on this value, but since the underlying data is still total quantity it will sort on the total sales quantity and not the difference between the sales quantity.

Any suggestions on how to get excel to sort the way values are displayed?