Are you basing that on anything — or purely speculating?
Because without any basis I imagine any negotiated terms are favorable to Trump, not Mulvaney.
That said, numerous sources indicated that Mulvaney was Trump's first choice. This was reported right after Allen's resignation was announced. Shortly after the mention of Mulvaney as first pick, Mulvaney went public with the statement that he wasn't interested in the job.
Then numerous other potential candidates stated publicly that they weren't interested in the job, preemptively ending further speculation.
Conclusion from the above and including what we all know about the job, potential candidates weren't stepping all over each other to get to the front of the COS line. The job is one of the most difficult and thankless in the administration. In addition to the normal difficulties of the COS job, being COS in a Trump administration is a particular challenge. The job entails instilling discipline and order in the WH staff and that is made more difficult by a president who himself enjoys and indeed needs a lively, chaotic, creative personal space from which to operative.
There were many reports that Allen and Trump wrestled over Trump's freewheeling style and his apparent off the cuff tweet habit.
Conclusion: I think it is safe to say that Mulvaney would want to come to a very clear understand with the President as to how he, Mulvaney, would balance a freewheeling and creative president on one hand and create a disciplined and orderly staff environment on the other.
This would come out of discussion and negotiation. As an aside, both Trump and Mulvaney understand negotiation and its importance in management.
As a comment on the President's style, I happen to understand his need for what is called "chaos." I don't think it's chaos at all. Trump is a creative thinker. Creative people need stimulation, not order.
Things in our world change extraordinarily fast. Trump can stay on top of them all. He even likes it that way...juggling many balls in the air at once.
That's how he keeps his enemies and opponents back on their heels.
Serving as COS for a manager like that is the ultimate challenge, so yes, I think Mulvaney did a lot of negotiating to reach an understanding with Trump how things would work if he, Mulvaney, accepted the job.
It would be a pretty safe assumption that the "acting" title means something has yet to be decided in this "deal" (i.e., negotiation are still underway).
That's my thinking.