Not understanding laws of supply and demand

I entered a convenient store today to make a delivery, and noticed they had a request for a credit. I had received no special instructions from my office, so I took the credit slip in and asked the store manager about it.
"Yes," he told me, "It's actually a trade. We've been waiting for the birch beer, and so we want to exchange it with a case of the RC cola you brought last week."
"Unfortunately, we're still out of the birch beer," I notified the manager. "We've been waiting for it to come in for almost a month."
Disappointed, the manager asked, "Oh, so you don't have any birch on your truck?"
"No," I replied.
Mr. Manager said, "I don't understand. Your salesman, 'Mark,' told me we could do an exchange."
"Unfortunately," I informed him, "Mark forgot to check with the office to see if the birch was in stock." Bewildered, the manager asked me, "Why would the salesman not do that?"

Hmm... An excellent question indeed...