Does a 20-year retirement from the Marine Corps goes toward a Federal Job (example working with at the VA)?

Can you still pull your retirement check every month doing so? I am currently working at the United States Post Office. I was under the impression that you could through them, but now since being hired I am being told I would have give up my retirement check and buy back my 20 years (which they told me I would be crazy to do). Is being a Federal Employee different? I know if I hadn't retired from the Marines; I would have been benefictial. But I don't see why I'm "sort of" being punished for serving 20 years for my country. (My retirement check is only about $1800 a month with a 40% disability through the VA.) Also does disability count as something I would have to give up also for either a Federal Employee or working for the Post Office also? Thank you and Semper Fi!