I think i miss understood your original question. My building is a pole barn. I consider it wood framed. I think you were talking about 2x4 like your existing building. The more I think about this the more I like replacing it the same. Sure you have to paint it but it matches the house. Unless you are trying to get away from the maintenance?
He can replace it anyway he wants it. Can add money to it can cut costs and still get the 35-40k he is receiving.
Basically he will be paid so much it’s his choice to make it any color,style. He could go with a Mansard style building A lean to type building a pole barn or a conventional 16on center garage sided with metal wood or vinyl siding.
He could also put it anywhere he wants on property etc etc .
Also matters if contents inside garage were covered . Anything scratched dented etc ruined he would be paid for as well. That’s where putting tarps over things gets you paid .
Clean up dumpster all should be paid for . Scope of work should also have overhead in it for contractors.
Whom ever builds it need to see scope of work what they are paying and go over it well I’m sure they are cutting costs somewhere. A good contractor would be able to point out hey your not paying going price for this material or install etc etc.
Maybe th concrete cracked some where on floor etc etc does it need to come out . Etc etc
Scope of work is where a fella gets started .