Property line disputes can be a huge headache as they can get real personal really fast. You may want to look into prescriptive easements, you can read up more about how they work here: http://www.moeland.com/adverse.htm
Presciptive easement? You have no idea if this fence was put up last week, last year, or last decade!!!! This could be a neighbor who didn't bother to properly identify lot lines before constructing a fence and now the encroached neighbor is demanding they move the fence to the lot line. No information was given that indicates a prescriptive easement may or may not be appropriate in this person's circumstance! If the person posting just recently built without doing any homework FIRST to make sure it was placed correctly, a prescriptive eaqsment may be a very inappropriate suggestion, AND ADVISING TO PUT THE FENCE WHERE IT SHOULD HAVE BEEN BUILT MAY BE MORE APPROPRIATE..
Poster, to begin with, encroaching on whom, and who are you in this., and how long has the encroachment existed? Have you reviewed title to see if it is addressed? A lot more info is needed to appropriatly respond.