Hey thanks for the advice! I'm always open to learning from others. The post is wood actually so maybe I'll reconsider the material I use to support it. That makes sense as I once learned you can't put a wood floor on top of concrete slab because of this very reason (the moisture issue).
Perhaps filling the hole with a mix of gravel and dirt would be a better option for long term stability (or maybe I should just get a metal one and use concrete with it as you suggested).