Credit Report

MY husband and I have been working on our credit report for a little while and have all of our past debts paid in full. Now all we have is one more left. Our problem is i called to settle the debt and they have no record of our account what so ever. I have given them our S.S.# and everything i possibly could to get this debt settled. Now this debt is the only thing that is keeping us from getting the house we want. How do i take care of something that i can't fix now. I tried to make our wrong a right but now i can't. The debt is a repo through Capitol One auto finance. Will the credit reporting company's take this off since Capitol One has no record of it in their system. If not do you have any advise for us
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dispute it fast
certified delivery mail
send it next day air if your in a hurry
say not yours
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I'm sorry to bother you again but how much will disputing this help our credit score.
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if it falls off and it was 2 years or less old
and you have new current rated credit
as much as 50 points
otherwise probably 10- 20