I would like to add DHL to the maybe list
I may have to check but Im sure the cargo slots come under a different set of permissions to passenger ones. Case point is the CX37 freighter into LHR, back when CX were looking to add more passenger flights to LHR, it was suggested that the CX37 could move to MAN with the other freight flights but was not feasible as the slots ''could not be used for passenger flights''. Just like LHR hold back some slots for private flying and diversions. I may have mis-understood what I was told but in other words DHL cannot sell their slots to say, BA for passenger flights. It could be down to where they park and terminal capacity, Im not sure.
I re-iterate I may be totally wrong on the above though.