differential

/ˌdɪfəˈrenʃ(ə)l/
(数学中的)微分;(经济学中的)差价，差额；In mathematics and economics, a differential is a difference between two values in a scale.

Germany and France pledged to maintain the differential between their two currencies.
德国和法国承诺保持两国货币之间的差别汇率。