First of all, I want to point out that the Ranking is not for all the Clients but only the top 100.
And now I'm going to explain you my guessing of your intention: Becasue I want the Sells of each client relative to all the sells in the group (not only the ones of the sells of the top 100 clients in the group), I changed your expression for:
= SUM ( AGGR ( SUM ( Sells ) , Client ) )
And it didn't work...
One question that came to my mind, by saying the AGGR forces the expression to move outside of the dimensions of the chart, do you mean ALL the dimensions of the chart or only the one you're AGGRing?