Sorry, but much like the saying "The early bird gets the worm", in this case "The first one to get their paperwork in gets to buy".
The sales manager doesn't know the unit is sold until he gets the paperwork. Turns out there were 2 salesmen working on selling the same unit at the same time, and yours was a little slow in getting it done and, obviously, took more than 10 minutes to complete it. In the meantime, the other guy got his done and submitted it. It happens sometimes, doubt you're the first one, nor will you be the last, either at this dealer or elsewhere.
Sorry, but I don't see this as a bad practise whatsoever. Their job is to make the sale, doesn't matter whether it's to you or someone else.
