I think you will get your dough. Since I work for a university, it sounds like someone in the system did not understand what the fark they were doing. POs are required when a price exceeds a certain dollar figure. Unfortunatey, they may have tried to pay you via check and found out they had to submit a PO. I would be pissed, but I do believe you will get paid in a couple of weeks.
FYI: At my institution POs are required when a price exceeds $2500