Copy and paste the below template (customise areas highlighted).
I understand $X00,000 is less than you want, but my fear is that our next best offer could be lower, and I don't want you to lose a good buyer who may go higher with competition.
If we turn this offer into a starting bid with Openn Negotiation, the bid can attract new buyers without letting the current buyer go. If there's a better price out there, this process will find it. Check out this video of how it works and let’s talk https://vimeo.com/316933951