"matter of negotiation" is correct and usable in written English.
You can use it to refer to something that requires discussion or agreement between two or more parties. For example, "The terms and conditions of the contract are still a matter of negotiation."
Just how much is a matter of negotiation.
Everything is a matter of negotiation and making some sort of adjustments.
The switchover to new technologies is not mainly a matter of negotiation but of engineering, planning, financing, and incentives.
Increasingly, they say, retirement is becoming a matter of negotiation, requiring new flexibility in setting workers' schedules and benefits.
Even though it's a question I hear regularly, it's not entirely accurate as a matter of negotiation vocabulary.
This was not a matter of negotiation between two equal parties; rather, the Nazi regime was dictating how an American business could create and distribute its product.
"The timing of civil and criminal proceedings is never preordained, but typically a matter of negotiations between the S.E.C. and federal prosecutors," said Eli J. Richardson, a white-collar defense lawyer at Bass, Berry & Sims and a former prosecutor.
But a spokeswoman for Mr. Silver said the governor's bill was still being studied and the matter was still the subject of negotiation.
The Turks see EU accession as a matter of genuine negotiation: if they make concessions, they expect concessions in return (eg, on northern Cyprus, see article).
"His point of view wasn't that rules aren't important, but that how they get interpreted and applied is a matter of mutual negotiation.
With only a handful of plausible internal candidates, pay was essentially a matter of bilateral negotiation between the board and the chosen.
