I could think of a couple of ways to do this.
1. You create a new section under detail,,, defaults to detailb and copy the details from detaila to detailb so that they are the same. Insert your paid watermark in detailb. Make sure you underlay this.
Now go to section expert (right click on the left gray area) in the detaila section and check suppress. Now on the right of suppress there is a x-1 button and put in a formula.
(invoice amount field) - invoice paid field = 0
This will suppress this section if = 0
right click on detailb and do the same but this time
formula says
invoice amount - invoice amount paid > 0
This will suppress this section if there is a balance.
You can go further again by creating another section detailc and do the same but this time
invoice amount - invoice paid < 0
Put a credit balance watermark in this one.
2. My other solution requires database manipulation but this one should work perfectly