They want me to pay for the mistake of their collector...

Their call Centre agent called me and told me that I qualify for 50% discount on a debt I’ve already settled with their client and has been updated as paid up on my Credit profile. Because I wanted their harassment to stop I agreed to settling at 50% in 7 days. Then I get a different balance on the Sms and when I called to ask what is going on, I’m told that the collector made a mistake and that I need to settle their full costs. And may I add that because I settled with their client, they added the commission they would have received from the client, to my account. I’m willing to settle the account, but only at 50% as the collector advised me. These costs have no impact on me, however I’m tired of the calls hence I’m willing to pay only 50%. If I don’t get a response in 8 days, I’m taking this to the CDC, as that collector recorded herself telling me about 50% and even confirmed the amount 4 times...