Hi Virendra,
I replicated the scenario in the following steps,
1) Created a PR from project for 4 quantity
2) Created a PO wrt this PR for 4 quantity
3) Did the GR for 3 quantity
4) Then changed the quantity in CJ20N for the material to 3 and saved the project.
5) Checked the PR in ME53N and PR quantity changed to 3.
6) In ME22N changed the PO quantity from 4 to 3
7) Checked CJI5 and i cant see any remaining commitment.
It worked ok for me. Hope this is what you did.
Regards,
Gokul