x1 (currency) is change for x2 (currency), with x1 having units x3 (collection of values of currency involved).
Change is a collection of currency (x1) which has the same monetary value as another collection of currency (x2) such that some of the currency involved is different (usually the units of currency are each of lesser monetary value than those being exchanged from x2). For example: a single bill worth 20.00 can be exchanged for four bills each of which are worth 5.00; the latter collection is change for the former (the single bill); both collections have the same total monetary value, but some of the currency (material item encoding or possessing monetary value) in the latter each has lesser monetary value than some of the currency in the former.