I wouldn't be all that quick to sign that letter.  I would certainly not do that without talking to a lawyer first.I hope that when you bought your home, you got title insurance (if you had a mortgage, the chances are good that you did).  And I hope that your car port doesn't extend out beyond that original foundation.  If both of those were true, I'd let the title insurance company handle the problem.  The bottom line here is, if your neighbor thinks you're wrong about the property line, let him prove it.