The intuitive perception of the complexity of the situation is largely accurate. There is a lost soul spirit attachment with a very deep karmic tie to the host client and it is based on high-level psychic attacks launched in the past. That is what is being seen in the nature of a kind of contract and what needs healing in order to allow this spirit to be moved on successfully. At present, there is quite a logjam because both parties are holding onto the relationship, although for differing reasons. The primary energies holding them together are karmic and that will require deep karmic repair beyond simply dissolving the agreement.
This is the number one complication with straightforward simple spirit removal procedures, that when deep karmic entanglements are present it can be a formidable challenge to pry these beings apart because there is a considerable investment energetically tying them together even though it is mutually self-destructive. This is the virtue of the Lightworker Healing Protocol, that it is geared to address such complications quite effectively. This would be the next best step, to bring your Protocol to bear for this client and facilitate a deep karmic repair for this unhealthy relationship between the living client and the spirit being who has a deep karmic history with him.