I don't know... it could just be a guy doing a favor for someone who made a BIG sale.
Ever hold the door for a complete stranger? Ever tell one, as you're leaving somewhere, something worth knowing about as they enter?
Edit. I think it's reasonable to say this was an example of ungoverned reasonable men being reasonable. Technically, they could have complained that Li had promised them the whole 'berg, and demanded a balance of its volume, but Li did them a favor, and their purchase was one of an approximate need. A few gallons more or less was of no consequence, and they weren't put out by its "loss". Li had another customer come up and had the opportunity to make a small profit, (the difference between the discount price and the regular price,) please the second customer, and do a favor for the promised first customer.
If you looked over and saw that your neighbors lawn was looking a bit shaggy...was just about to pull out the lawnmower anyway, just as a favor, and someone told you they needed about 10 gallons of anything organic, so long as it'd decay properly for a science project, and they'd pay 10 bucks for it, what would you do?