Let's say I'm a disgruntled McDonald's employee. One day I walk up to someone and give them a piece of paper saying "McDonald's agrees to give you ONE MILLYUN DOLLARS in exchange for your pocket change. Signed, me, on behalf of McDonald's."
Now, I really doubt that this contract could be enforced against McDonald's in court. Presumably their articles of incorporation specifically set a list of people with the authority to enter into contracts like this, and presumably that list included only high-level directors and such. Yet clearly other people in the organisation have some authority to enter into contracts, for purposes ranging from signing payroll checks to buying supplies.
So how is all this regulated? What determines what agreements a given employee can enter into on behalf of their company, particularly if there aren't any specific provisions in that person's employment contract?