I think you should take a deep breath
You can't really blame a young teenager for trying to get as much money as possible ... they do not develop empathy at this stage but think only of self .. in his mind he was getting a great deal, tough on your child .. I think that's rather natural and not really bullying TBH (he has learned the basis of the free market economy we live in: things are worth what other's will pay for them)
You can't really scream and shout at him in front of his friends, it is a highway to nowhere
What you can do, and should do IMO, is talk to his parents and say you were absolutely livid although you appreciate it is natural development that you would be grateful if they would talk to their child because you feel rather as though your child has been mugged.
I think it is fine that your child is taught the value of money .. but the teenager has merely learned the value of the free market economy which is that things are worth what others will pay for them ..