That is a massive issue
I don't now what the load on that post is but I know they don't just put them in for show. That's an integral structural member have your support system in your house. The fact that the Builder says it's okay would scare the crap out of me. I wouldn't even consider buying that house because that statement speaks volumes of the builders quality of work.
This is what a footing is supposed to look like for the structural postwww.cottagelife.com
Footings will vary in size depending on the ground conditions. In my area our soil is mostly clay or sand we don't have much in the way of aquifers and we don't get sinkholes in the middle of the street. a typical size for a foundation in my area is approximately 24" by 24" by 1 foot thick, reinforced with rebar. The biggest one I've ever seen was 3 feet by 3 feet and two feet thick reinforced with rebar.
Keep in mind that the ground didn't rise around the post. If that were to happen it would have just pushed up the house. Your house has sunk and everything above that post has dropped along with the it.
First you need to have an engineer tell you how big of a footing you need and the engineer should also be able to tell you the method for repairing it. Usually it would involve something like, building a false wall on either side supported by multiple adjustable Jack posts where you would jack up the house to the correct height. You would then cut the concrete excavate the appropriate amount of dirt, gravel or whatever and then pour the appropriate footing. In my neck of the woods structural member designs need to be certified by somebody in case something awful were to happen. A finger needs to be pointed at somebody. The engineers and architects design a plan and give it the stamp of approval. If that plan is followed and something awful happens they are responsible, if the plan was not followed then whoever did the work is responsible. For some reason city inspectors don't often get the finger pointed at them even though they pass the inspection.