Automatic deduction after UD & Quantity reversal

Hello,

I have this scenario and I'm supposed to implement this. This is related to Incoming inspection where QM is integrated with MM module.

There are two issues which I'm not able to address.

A GR quantity is done for 100 pcs. After Quality inspection, there could be two possibilities,

1. If 20 out of 100 pcs are rejected & posted to Block/ return delivery, there should be automatic payment deduction. Payment need to be made only for accepted quantity i.e 80 pcs.
2. In another scenario, if user mistakenly post some quantity to returns and saves the UD, how to revoke it? I am not sure if UD reversal program would bring the quantity back in quality stock. Is there any other possibility to bring this quantity back into quality stock?

Pl help me in mapping this scenario considering both the possibilities.