Running Bill (Advance adjustment)

Hi,

Need to resolve a situation , where the customer has paid certain percentage (say 20% of total agreement value) as advance.Now , we want to adjust this percentage in every bill we raise from Projct Billing.E.g.The invoice lines should look like :

1. Total Billing amount 200000

2. Advance Adjustment (20%) <40000>

3 10% Retention <20000>.

Is there a standard functionality in project billing (R12.1.2) to do so ? Advance on agreement doesn't seem to help ?

Regards

Gagan