Thinking of how I've finally paid my loan, I told myself that from now on, I'll get extra "free cash" each month. But this idiom always made me curious. The fact that both words contradict each other gives it quite a comical effect, to which it is often used. How and when did the idiom "free cash" come to be?
EDIT: It seems one corner case is not addressed by the actual answers. if the terms is related to taking money from an ATM or taking a loan without interest, why does some comics or cartoon character use the idiom when finding some money that they didn't earn (i.e. found on the street, in the house of the neighbour, given as a reward,...). Unfortunately my foggy memory can't replace it in context.