Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to negotiate a job offer in writing, use this factual base: state the role, policy, decision, dates, and process that apply without disclosing another employee's information.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Explain the available next step, support, review route, or deadline and name the correct contact. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not imply a guaranteed outcome, retaliate, or use informal pressure where a formal process is required. Use HR, a representative, legal advice, or emergency support for discrimination, harassment, threats, or immediate danger. You do not need the other person to agree with a boundary for you to follow it consistently.