Commission calculation in Incentive Comp based on Receipts / Payments in AR

Am trying to find a solution on how to calculate salesrep commission based on receipts in AR.  Commission will only be calculated on receipts if the salesrep achieves monthly sales target which is based on the AR invoices.  i.e. if total of invoices is 100 K then salesrep is entitled to commission on the receipts.

The problem is that normally the receipts are collected in a different month to the invoices.  Salesrep can achieve invoice sales on 100K in Dec-08.  Commission calculation for Jan-09 will may include receipts for Nov-08 and Dec-08 invoices.  For each receipt we need to check which month the invoice was raised in and check if the salesrep made the target ?