How to partially pay a bill on a purchase order

How to partially pay a bill on a purchase order

I am looking to start listing my purchase orders placed to my suppliers. Our current arrangement is to pay a 50% deposit then the other 50% on a net 45 day after delivery. The issue I am running into is that when I convert to a bill, it then marks the PO as closed, even though its not. I see a filter on the list for partially billed but don't see a way to enter a PO into that status. Any help on how to achieve this would be much appreciated.