Urban Outfitters — attempted theft - extortion

My son got caught 'attempting" to take two shirts from Urban outfitter's - He never left the store and put the shirts back. he is 20. they pulled him in the office, asked him to sign a form stating he would not return to the store for three years or he would be charged with tresspassing. However, a collection agency has contacted him and told him to pay $625.00 or they would pursue a "Civil" case against him. He paid $140 before we even knew about this situation. Now reading blogs, appartantly this Palmerpay.com collection agency bullies by threatening them with suits to pay. They have no proof & he did not do anything wrong. What can we do to get back his $$.