SA
You are entitled to fight each charge against you on your credit report. They are required by law to substantiate each claim, not you. If they have evidence, you are allowed to contradict it with evidence of your own
Practically, if you owe the debt, what you can do is make an arrangement with the person you owe to not only remove it, but not cooperate with the credit agencies when they attempt to verify the claim. They cant lie and say you paid it on time, but they can choose in writing not to respond to inquiries questioning its validity. If they refuse to do so, the credit agency has to remove the negative item and show it as non verifiable. That will help your score substantially.
Hope that helps
And rememebr, its useless unless they put it in writing