Once everything attached to the girder is supported properly you could lower/remove the post. The girder may still be attach from the flooring nails/screws above. You may need some elbow grease to pop it off. Any screws protruding through the subfloor should be ground off of course. If your you use a grinder to cut the nails make sure you wet the area lightly with water to extinguish sparks before and after.
That is bad! I hope every one read your post. Never ever notch a girder. unless you have a engineer sign off on it. It might behove you to talk with a engineer and have them run the calc on it and see if it OK to do so and save you the headache of removing and putting back in that girder.