Discrepancy in advertised lot size and actual lot size

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Hello, I am interested in this property in Seattle, WA.  After doing my due diligence by looking through public records, I have found out that there is a huge discrepancy in the advertised lot size, the tax record size and the actual size by 40%. Apparently,  there was a dispute with a neighbor 25 years ago. The previous owner and the neighbor re- drew the property line. The previous owner sold the property to the current owner without updating any public records. I have not made an offer yet. What is the normal course of action in this case if I were to make an offer? How can I use this information to negotiate the price down?