先说断后不乱xiān shuō duàn hòu bù luàn

reachagreement first so as to avoid future disputes
❍ 官人在上，不当老身意小，自古～。(《金瓶梅》七)Esteemed sir,not that I as an old woman am cautious,but that agreement must be reached first so as to avoid future disputes as it has been practised since ancient times.