Applying GST on Milestone Invoice in Oracle Payables

Hi Team,

We have business scenario, wherein we need to make milestone payment to suppliers as per the agreed milestone activities stipulated in PO thru complex Purchase Order. Such PO has advance and recoupment where we need to adjust the advance amount in the subsequent milestone bills.

Milestone Payment:

     1. 20% Against submission of ABG

     2. 40% Against Supply

     3. 20% Against Installation & Testing

     4. 20% Against Acceptance / Operations

               a. Retainage 10%

How does the GST works in this scenario?

Supplier may not provide any tax invoice for Milestone 1 i.e 20% against ABG. But will submit ABG document. How does the Invoice booking and Payment works here and how the tax gets applied here