If the cops were not trained well enough to at least recognize legal tender then Baltimore PD has a very huge training problem. Best Buy's training program is a joke as well it seems. The manager should have known $2 bills are legal. It's his job to know. He just opened up Best Buy to a false arrest law suit.
The article is not clear on the specific issue; the ink ran on some of the bills, so even if they were $1 bills the store might have called the police, thinking that they were just printed, with the consecutive numbers.
My guess is “smeared ink” is one of the things they are trained for, and consecutive bills. My issue with this is mostly that they actually arrested the guy. They could have gotten his information, and let him go, and then investigated. I mean, he was a customer. He clearly lived in the community, and wasn’t going to go on the lamb.
So if you check the bills and they are counterfeit, you know where to find him. Just lazy cops thought it would be better not to have to worry about finding him again.