Client unable to provide proof

My son's credit report showed an amount of 275 as unpaid debit posted by a collection agency. Upon following up with the agency, we came to know that it was from his previous rental place. We followed up with the rental agency, they were unable to provide evidence as to why the charge was made. The rental agency agreed to advice the collection agency to resolve this as non debit. While this happens, can some one please advice as to damage claim can we make to the collection agency or the Client? It seems that this negative charge was on my son's CR for a while and he was infact paying higher interest on all - his credit cards, insurance etc.