Simple credit question

In a phone conversation with Bank of America I was shocked to learn that some collection agency has me owing someone $365.00 from 2005 that has not been paid. Otherwise I am blemish free, but this black mark has by credit score at 676. I know my score was 720 in Jan 08.I feel sure this is the work of my ex-wife. How do I pay the bill & get rid of black mark?
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dispute it as not yours
if they find it
legally validate the debt
if they can do that
negotiate to pay if they will remove it
you need to know if thery have your signature and legal chain of ownership