TL;DR

The project sponsor has a significant number of expectations. In the beginning, they will try to make all expectations to be project drivers. Here is what the project manager could do. The post is based on a remarkable book written by Johanna Rothman, Manage It!

The project sponsor has great expectations:

This project must be done in three months with this feature set and with a low defect rate. Use these six people with a budger of 200 000 US$.

As a project manager, you need to negotiate only one project driver that will be fulfilled at the end of a project. So you must say NO to the above question after you have done the initial estimation. Running the estimate is essential, so you have back up evidence data that five drivers would cause project failure.

Ask the sponsor to prioritize all expectations. The expectation with the number one priority becomes a project driver.

Johanna warns us what usually happens when a sponsor does not make a decision. That decision falls back on the project manager again, and the project manager transfers it to team decision. And every team member would have a different driver. Sounds familiar?

Conclusion

Convince a sponsor make a priority list of their expectations. That decision is on the sponsor, not on you or other team members.