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This would be a good start, but could be tricky to apply. On the external engagement, accountability has more teeth ($$), whereas internally, you both live in the same house and have to continually make things get through.
Have you identified the key issues surrounding the handoff? Is it scope creep or some other nagging issue? Ultimately, the real use of such a document or policy is to refine and improve the sales handoff process, rather than control it. Show how many times they change the game - and use it to prevent handoff issues on future engagements. Hope I've communicated this well enough.