Bava Batra 170: Practical Cases of Contested Loans (plus)

When an apparently previous owner claims he is the current owner, against the apparent current owner. What kind of proof would be necessary? All of which leads into a case and a story - to summarize the issues and the legal decision: an outstanding loan that is contested. Also, a new mishnah! The case of one who has paid a portion of his debt, what do you do with the IOU notes? How much scope for confusion - or fraud - and when do you need the courts?