Is this supposed to be possible?
Yes, all buildings can have a negative "jobs available" figure: passengers are generated in packets of several passengers at a time, and all of them will go to a destination if there are jobs available there if their trip is a commuting trip. The number of available jobs will be diminished by the number of passengers arriving. Because of the time between passengers departing and arriving and the fact that multiple passengers can be treated as the same packet, more passengers may arrive than there are jobs available, which will send the number negative. Because the number may go negative, however, more passengers arriving than the building has jobs does not affect the balance, as the counter will take longer to get to a number greater than zero at which point further passengers can again start their journeys.
I had planned to change the UI so that it shows 0 when the number is negative before I produce a release version, but showing the negative numbers is useful for testing the passenger generation system.
Ok, thanks!