So you want your sum or partial sum formatted differently than the other values? I'm not sure there's a simple, direct way. However, you should be able to use dimensionality() to accomplish the same thing. Let's say your pivot table has two dimensions. Then the dimensionality of the detail rows will be 2, the partial sum will be 1, and the total will be 0. So you should be able to do something like this (untested):
num(sum(Value),pick(dimensionality()+1,'#,##0','#,##0.00','$#,##0'))
And make sure you don't override the format on the Number tab.
