Not much to add. I agree that it depends on what is in the contract.
Just one question. It sounds like the GC works for the company that supplied the lumber (you said he is a rep of theirs)?
If the GC claims that the extra lumber is "his", is he planning to bring it back to the lumber...
I'm looking to build a 2 car detached garage. I currently have a single car garage in the location. Should I be pouring a new slab, or extending the existing one?