You won't be able to do this using a parameter as parameters unfortunately can't be populated dynamically based on other values. It sounds like the easiest solution to what you want to do would be just to create a quick filter using the Sales Date - although this would hide the non selected dates.
If you need the non selected dates to remain visible then you'll need to keep the chart worksheet of your dashboard and Top 5 piece separate with regards to sharing (making global) the date filters. In other words use the date filters on the dashboard that reside on the Top 5 worksheet.
If this doesn't make sense or I'm over simplifying things then please post a workbook.