Retainage in Service Billing module

Hello,

I have set up a 10% RETN rule in P48128.

I have created my invoice ($5000)  thru Invoice Generation and when I look at R48300 for the GL entries, I see the credit to revenue accounts and a Debit to A/r ($4500) trade and Retainage.($500)

When I look at P48250 ( invoice History)  I see the retainage amount there .

How do I Release this retainage and invoice out to the customer thru the service billing module?

thank you