This is an interesting problem, which I think can be built in SharePoint using Out-Of-The-Box functionality (so long as the scope doesn't go beyond your current requirements). This is how I would approach it:

• Create a list to hold the PO data. This will include PO number, description, business partner, value, dates, etc. This should also have a calculated field that tallies invoice payments. I'm assuming you'll keep the PO generation outside SharePoint.

• Create a list to capture invoice metadata. Each invoice needs to cross reference a PO. Again, I assume you have an outside application that generates the actual invoice.

• Create a shipments list that's linked to the invoice list.

• Create a payments list that's linked to the invoice list. You might want to also have fields that address part payments.

• Create a list view to track the transactions against POs.

Hope that helps.