The option I usually use in PR approval workflows is a decision task:
If the user approves - I use the release method (background)
If the user rejects - I send a task to the initiator/buyer (usually a responsibility rule based on the purchase group) this has no effect on the status of the PR. no need to change it since it's not released.
if you don't want to use a rule you can add a task of selection of the recipient at the rejection branch.
if you are using the direct release by the user in ME54N then the task will be completed by an event (release or reject). To the best of by understanding in this case the dialog process chain option will not work and you will need to find the recipient in another way (initiator/rule/partner function in the PR etc.)
