In SSRS 2005 I think you would need to have the sales values for different dates returned in the same row in the dataset. So that involves modifying your query possible using PIVOT or something like that.
In newer versions you can refer directly to other cells in the matrix using this:
You could create an expression in the format property
=IIf(Fields!Column.Value > ReportItems!TextBoxName.value,"Green",IIf(Fields!Column.Value < ReportItems!TextBoxName.value,"Red","Black"))
The formatting formula would be similar if you have to use the first option, it would just refer to two fields at the same grouping/details level.