The issue at hand is that OpenAI (the private, currently "capped profit" entity, henceforth in this post referred to as OpenAI-P) is trying to get spun out of control of the OpenAI non-profit (henceforth referred to as OpenAI-NP).
In order to do this, OpenAI-P must compensate OpenAI-NP for its ownership of OpenAI-P. Currently they are trying to value this at $40B. This is a swindling for numerous reasons. Firstly, OpenAI-NP has complete control over OpenAI-P. They OpenAI-NP board can do whatever it wants with OpenAI-P. OpenAI-P has some financial obligations to those who have purchased its PPU (its equity instrument) but ultimately OpenAI-NP controls OpenAI-P and could do whatever it wants with it (eg shut it down, fire Sam Altman on a whim, etc). Further, the cashflows that OpenAI-NP still has ownership of are, at least under the claims of people like Sam A, clearly worth dramatically more than the $40B valuation would imply.
In addition, the OpenAI-NP mission is "Our mission is to ensure that artificial general intelligence—AI systems that are generally smarter than humans—benefits all of humanity". The control of the leading AI Lab is the only way that they can possibly live up to their mission. To abdicate control of OpenAI-P is to abdicate their mission. If the OpenAI-NP board follows their mission as they are obligated to, they cannot possibly be willing to abdicate this control and they must surely value it far beyond $40B.
EDIT TO ADD TL;DR - Musk is trying to force OpenAI to value OpenAI-NP's stake in OpenAI-P at a reasonable level which is far greater than the current $40B which will help Musk by potentially derailing OpenAI-P's goal to be spun off.