di'ardu'edi'i lujvo

x1 is the excess money/change (that should be) returned for/after/with the purchase of x2 by purchaser x3 from vendor x4.

Beware sumti-raising. If the ownership of an item costs 9.50 dollars and x3 buys it with an acceptable bill of value 10.00 dollars, then the change (x1) is the monetary value of 0.50 dollars (regardless of which bills, coins, etc. carry that value), where 'dollar' can represent any monetary unit.