How do you negotiate an offer-deadline extension without looking unprofessional?

Ask once, with a concrete reason and respectful tone.

1. Thank them for the offer.
2. State the current deadline.
3. Explain the specific timing conflict.
4. Ask for a reasonable extension.
5. Acknowledge that you understand if it is not possible.

'Thank you again for the offer. I am very excited about the opportunity. My current deadline is Friday, and I have one final round already scheduled for next week. Would it be possible to extend the decision deadline by one week so I can make a fully informed decision? I completely understand if your hiring timeline does not allow this.'

threats, repeated extensions, vague reasons, or using the extension to shop the offer aggressively.