OK - that's a bit more soecific, but I'm still not clear on exactly what you need here (remember that it's 3:30 in the morning wher I am, so that might be entirely my fault!). do you mean by:
"A sale is made and a % of the gross profit is commission but only paid when the invoice is paid is there a way that when the date of payment is made that the commision figure for the current month is
If(monthof_invoice_date_paid+thismonth, then calculate cumulative commision)...
or something like that?
a detailed example would include at least your starting data & the expected result, not just a schematic of the fields...