I am in the United States military and was charged with underage drinking, my penalty was to be served out through 50 hours of community service by the following year, which makes the deadline only a few days prior to my 21st birthday. Is it possible for me to make a payment for the amount of community service I owe?

You will have to ask the court, but normally no, normally they want you to do community service to teach you a lesson and help you learn a lesson.

Also normally they will not release you from probation till it is done, And you can even have probation revolked for not doing it.

Why are you not doing such a small amount of community service

I was in charge of court-ordered community service workers at our public library. If you're having trouble finding places to do community service, please ask. Like Fr_Chuck said, fifty hours shouldn't take that long to do. Most of my workers had to do over 200 hours within a year or less. And yes, the hours assigned are to teach you a lesson and to help you pay back the community that you shamed with your underage drinking.