I'd like to run a custom monthly report that shows product/service & the payment received in the month (not sales) for that specific product/service. In my case, I sell a service that is often paid for over a few months. I'd like a summary of payments received by months. 

Example. customer signs up and an invoice is created for $1000 (i.e. a sale of Item A). In month 1 they pay 600 and in month 2 they pay 400. I'd a report that shows 

Month 1:

"Item A"   

Payment: $600

Balance: $400

Month 2 

"Item A"   

Payment: $400

Balance: $0

Also, I can a commission based on payment received (i.e. in month 1 it would be $600 multiplied by a commission percentage). Is there a way to create a custom field that multiplies the payment received by a set %? 

There are similar questions but the solution provided is sales by product service not payments. 

Thanks