There will always be someone superior to a manager. A manager will never be in full control of transfers or player logistics in general (contract renewals etc). There will always be someone to veto, no matter if this is owners, the board of directors, ceo's or dof's. Same applies to United and Mourinho.
For what we know, maybe Mourinho prefers someone above who is more football oriented than commercial oriented?
The main thing no matter what the title of the managers superior is, is that the manager and club shares the same belief and pushes in the same direction. If this is not the case, the manager will need to leave at some point. Simple as that.
City also have an DoF, it doesn't seem to stop them buying and selling whomever Pep points at. A DoF doesn't mean that the manager has no say in transfers anymore.