Shevuot (Oaths) 38 - Court-Imposed Oaths: Partial Admission
Courts have limited authority in imposing oaths, and can do it in only three specific cases: partial admission, oath of custodians, and the single-witness oath.
The first case, partial admission, has its own additional limitations. This oath can be imposed only if the claim is at least for two silver maot (volume of 32 grains of barley, about $10), and the admission is at least a perutah coin (half a silver barleycorn, under 25 cents).
In addition, partial admission must be for the same kind. For example, if the plaintiff says, "You have two silver coins in my possession," and the defendant responds, "You have only one perutah coin in my possession," the defendant is free from an oath, because the claim was for silver, and the admission - for copper.
Art: Thomas Waterman Wood - The Doubtful Coin