Service PR not editable after partial quantity is adopted into PO

Hi

We are facing an issue in case of service PRs. If a PO is created with reference to a service PR, the PR is rendered unchangeable especially the service tab is all greyed out. Our user has a case in which he created a service PR in which there was a service with quantity 100 in item tab. PO of quantity 30 was created by adopting this PR. Now no more service is required, so the user wants to change the quantity in PR from 100 to 30 but the service tab is all greyed out. Kindly help.