The only way I've ever been able to make use of Sales Commission in Sage is to capture the data (salesperson #, split percentage, if any, etc.) and then export it to Excel or Access at the end of the month. Or use ODBC connections, your preference. At that point, you just need to run whatever macros are required to give you the payment data you need. Of course, then you have to key the payments back into payroll or A/P.
Come to think of it, if you're using Sage Intelligence, that might go a long way toward streamlining your process for you.