Handling a child who repeatedly negotiates after receiving an answer

My child hears ‘no’ and immediately opens negotiations: what about just ten minutes, what about tomorrow, what if a sibling can do it? Sometimes the questions are reasonable, but I'm exhausted by every small decision becoming a debate. I want to allow genuine discussion while making it clear when a decision is final. Have you found language or routines that separate asking once for a reason from arguing indefinitely?