things like this varies between countries, but usually, unless its specifically stated in a written contract or agreed on beforehand, the creator retains the ownership, while the commissioner gets limited usage rights for the purpose it was commissioned for.
As far as I'm concerned if someone commissions you to make something then its theirs, source files and all. If you use something that you previously created, such as substance graphs for instance, then they remain yours to re-use on other projects, sell, etc...