If you are selected for promotion to E-8 and still want to retire at 20, is that possible without completing the 3-year promotion obligation?

I believe there was or is a stipulation that allows you to retire but I would just retire at the rank of e7 with a retirement pay of my last 3 years. I did some research and couldn’t find the answer in black and white. I am going to ask my J1 about this but figured I would ask the community first. Thanks ahead of time for any responses.