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There are two steps you can take.
First, send the collection agency a letter by Registered Mail. Set out the facts and include any documentation you have that shows the agreement. Give them a deadline to remove the report from you record and tell them if they fail to comply you will commence legal proceedings against them.
Second, use the complaint process provided by the credit bureau and there to send in the documentation that sets out the agreement and also give them a copy of the letter you sent by Registered Mail to the collection agency.
If you've taken both of those steps and neither has worked in your next step would be to retain a lawyer to send the collection agency and letter threatening a lawsuit.