PR Approver not allowed to change PR but PR Creator allowed to change PR

Dear Gurus,

My client has this requirement whereby only the PR creator (the person who has Authorisation to Tcode ME51N, ME52N and ME53N along with Activity - Create, Change and Display) can change a PR. They do not want any PR Approvers to change the PR when they access ME54N to approve the PR.

At the PR Release Indicator configuration, customization of the Field Selection Key for e.g. FZ01 or the Changeability to = 1 (Cannot be changed) even before any approval is given is not an option because it will block out even the PR creator.

I tried looking at the Authorization Objects for ME54N at PFCG but the moment I change any of the M_BANF* Authorization objects to Display only, I cant perform any approval at all.

Would really appreciate any advise which can help solve my issue. Thanks in advance.