I suggest a new feature/ parameter which, if active, allows the reviewer or user/ submitter of a PR to cancel a PR line and to add a PR line.

During the review in a workflow of a PR (not PR line workflow), the reviewer is not able to cancel or delete a PR line.
Hence, if the user has chosen the wrong item or purchase category, the reviewer can only reject or request change.
To 'reject' or 'request change' sends the PR back to the submitter but the submitter cannot perform changes beyond what the person who checks the PR can do: he/she cannot cancel or add a line and hence, not change the used item.
The whole PR needs to be recalled by the user, the line deleted and a new one created and only then, the PR can be resend to the approver.
It is hard to understand for any customer why a recall is needed, if the PR is already assigned to the submitter.

Also, in this case, the workflow history is lost. (See my other suggestion to change this.)

In a PO workflow a PO line can be cancelled or added by the reviewer (if changes are allowed upon review).

To unify the user experience between PR and PO workflow and to streamline the PR process I suggest the above mentioned function.