The phrase "subject to negotiation" is correct and usable in written English. You can use it to indicate that something is open to negotiation or discussion. For example, you can say, "The terms of the contract are subject to negotiation.".
What is subject to negotiation?
The bracketed passages remain subject to negotiation.
Those conditions remain subject to negotiation.
It should not be subject to negotiation".
"That is not subject to negotiation," Mr. Hoyer said Wednesday.
"All of the provisions would be subject to negotiation".
The deal is still subject to negotiation and public comment in the fall.
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Software Engineer at Adobe, UK