Jì / rán / nǐ / yǒu / yuē / zài / xiān， / wǒ / men / jiù / gǎi / tiān / zài / yì / qǐ / chī / fàn / ba。

既然你有约在先，我们就改天再一起吃饭吧。

Since you already have a prior engagement, let's have a meal together another day.