The terms are different in that, they shouldn't be demanding that you work X hours, but a maximum of X hours, and then a hourly rate for hours that exceed the set amount. For example, they agree to pay you $xxxx monthly for a maximum of, say 20 hours per month. If they go over 20 hours, they agree to pay you $xx / hr. If you work less than 20 hours, they still pay the retainer amount.