The scam is to accuse you of giving them the wrong note not a 30 Egyptian pounds note. The taxi driver gets aggressive saying you have given him the wrong money, and locks you in. When you open your purse or wallet to get more money out he snatches the money, waving it around saying this is real money. When he gave it me back he had stolen half of it.
Tip: Try to have some small money in a separate pockets before handing the driver the money make sure he and you both see the money that is going to be exchanged.