In order to add something to the product and stay on schedule, you have to give something up. I was once asked to add multiple colors and fonts to the text editor in a creativity program. I explained that I could do that, but in order to stay on schedule, I would need to cut the ability to move and resize pictures. Given this information, the client decided that a single font editor would be just fine. It's important that people working together are on positive terms.
Rapport is important.
If something unpleasant must be said or done, let your boss play the bad guy and do it for you. Don't let a disagreement lead to a personal issue between you and any of your partners on the project. When all else fails, say you can't agree to the requested change without adjusting schedule, budget, or both. Leave the decision up to the people in power requesting the change. People need to have choices and feel in control.