This post contains commonplace USA provincialism.

It is not clear that a contract is invalid because an
employee exceeded his authority when making it.

Being able to pull out a disclaimer signed off by a co-worker
or workers, especially for a sum, would muddy any actions considerably.
Even a small sum. A petty cash sum.
Whether that co-worker followed all the company procedures
in filing the contract, getting approval, etc. should
be moot. Those are issues between him and the company.

The same basic principles that give an employer rights
to your home work, bind the employer to the actions
that your co-worker takes as an employee.
This is basic. If a Wal-mart employee
shoves a mop handle into your herniated groin, you would
take action against Wal-mart not the employee.

Reasonable-man-being-prudent test would seem to be an issue,
for the co-worker to avoid problems.

If a company pursued action, an immediate & publicized
countersuit for breach of contract, or harassment would give many
corporations pause.

There was a scam where a disreputable plating company
would promise many coats and a finish like that on the
X pens. They would get a 'small sample order' from a junior
purchasing agent or whomever. The price would not be nailed down, just that
it would not be much. When the purchasing company wanted
to not pay the $50,000 to $100,000 bill, the scammers
would go to court for breach of contract and deadbeat-itis.
This was apparently a successful scam. I knew one
scammee, his fortune 100 company paid, they didn't want the publicity.