To track Retainage amounts in Estimate Rocket, each time you create an invoice with Retainage, create a new line item as shown below using the Invoice Retainage amount for the Price. You may also want to add a #retainage tag to the project to make them easier to find and report on.

Make sure and use the Group edit button to Exclude from Document so that the project totals are not affected.

Setting the Group Edit settings as shown below will show you the total for all of the Retainage Line Items:

Your Retainage Group should look like the image below:

This gives you a running total of retainage amounts which should equal the unpaid amounts on retainage invoices.