Commissions, Billing schedule and or part payment

I cannot seem to find any detailed documents on commission schedules, hence this post.

We have a commission schedule set up based on collections with the calculation set as marginal. We also have gone to billing schedules to handle our payment plans because we wanted control over the customer payments and there is no way to memorize customer payments. We now have a problem in that our sales reps/partners are not moving to the next commission level when part payments are made on a sales order, as per the billing schedule ie only full payment will contribute to a sale for commission purposes of moving the sales rep/partner up 1 sale. Is there any