According to SAP note 3053335, workflow rule actions that are scheduled “OnSave” are executed BEFORE the BeforeSave. Accordingly, the BeforeSave cannot influence the OnSave action.
However, SAP can enable a backend flag, called “Late Condition Evaluation”.
With that enabled, OnSave actions are executed AFTER the BeforeSave and therefore the BeforeSave suddenly has an influence on the OnSave action.
Some background information recently obtained via an SAP incident regarding the finalize logic (finalize means that BeforeSave is triggered):
In case you're facing a C4C dump (500 Internal Server Error), create an incident from it and find that the the Incident Context => /SAP/ABAP/RuntimeError file shows the error "Object instance creation of action rejection implementation class "<PDI solution namespace>_CL_ACTION_VALIDATION" of action "<PDI solution namespace>_<ID>" on node "ROOT" in BO "<BO>" failed" - you have to do a full solution activation to resolve the issue.